Improve expunged message/UID tracking in IMAP sessions, track synchronization history for mailboxes/annotations.

Keeping the message files around, and the message details in the database, is
useful for IMAP sessions that haven't seen/processed the removal of a message
yet and try to fetch it. Before, we would return errors. Similarly, a session
that has a mailbox selected that is removed can (at least in theory) still read
messages.

The mechanics to do this need keeping removed mailboxes around too. JMAP needs
that anyway, so we now keep modseq/createseq/expunged history for mailboxes
too. And while we're at it, for annotations as well.

For future JMAP support, we now also keep the mailbox parent id around for a
mailbox, with an upgrade step to set the field for existing mailboxes and
fixing up potential missing parents (which could possibly have happened in an
obscure corner case that I doubt anyone ran into).

webmail/api.json

@@ -1601,9 +1601,37 @@
 						"int64"
 					]
 				},
+				{
+					"Name": "CreateSeq",
+					"Docs": "",
+					"Typewords": [
+						"ModSeq"
+					]
+				},
+				{
+					"Name": "ModSeq",
+					"Docs": "Of last change, or when deleted.",
+					"Typewords": [
+						"ModSeq"
+					]
+				},
+				{
+					"Name": "Expunged",
+					"Docs": "",
+					"Typewords": [
+						"bool"
+					]
+				},
+				{
+					"Name": "ParentID",
+					"Docs": "Zero for top-level mailbox.",
+					"Typewords": [
+						"int64"
+					]
+				},
 				{
 					"Name": "Name",
-					"Docs": "\"Inbox\" is the name for the special IMAP \"INBOX\". Slash separated for hierarchy.",
+					"Docs": "\"Inbox\" is the name for the special IMAP \"INBOX\". Slash separated for hierarchy. Names must be unique for mailboxes that are not expunged.",
 					"Typewords": [
 						"string"
 					]
@@ -1665,20 +1693,6 @@
 						"string"
 					]
 				},
-				{
-					"Name": "ModSeq",
-					"Docs": "ModSeq matches that of last message (including deleted), or changes to mailbox such as after metadata changes.",
-					"Typewords": [
-						"ModSeq"
-					]
-				},
-				{
-					"Name": "CreateSeq",
-					"Docs": "",
-					"Typewords": [
-						"ModSeq"
-					]
-				},
 				{
 					"Name": "HaveCounts",
 					"Docs": "Whether MailboxCounts have been initialized.",
@@ -2165,14 +2179,14 @@
 			"Fields": [
 				{
 					"Name": "ID",
-					"Docs": "ID, unchanged over lifetime, determines path to on-disk msg file. Set during deliver.",
+					"Docs": "ID of the message, determines path to on-disk message file. Set when adding to a mailbox. When a message is moved to another mailbox, the mailbox ID is changed, but for synchronization purposes, a new Message record is inserted (which gets a new ID) with the Expunged field set and the MailboxID and UID copied.",
 					"Typewords": [
 						"int64"
 					]
 				},
 				{
 					"Name": "UID",
-					"Docs": "UID, for IMAP. Set during deliver.",
+					"Docs": "UID, for IMAP. Set when adding to mailbox. Strictly increasing values, per mailbox. The UID of a message can never change (though messages can be copied), and the contents of a message/UID also never changes.",
 					"Typewords": [
 						"UID"
 					]
@@ -2435,7 +2449,7 @@
 				},
 				{
 					"Name": "ThreadParentIDs",
-					"Docs": "IDs of parent messages, from closest parent to the root message. Parent messages may be in a different mailbox, or may no longer exist. ThreadParentIDs must never contain the message id itself (a cycle), and parent messages must reference the same ancestors.",
+					"Docs": "IDs of parent messages, from closest parent to the root message. Parent messages may be in a different mailbox, or may no longer exist. ThreadParentIDs must never contain the message id itself (a cycle), and parent messages must reference the same ancestors. Moving a message to another mailbox keeps the message ID and changes the MailboxID (and UID) of the message, leaving threading parent ids intact.",
 					"Typewords": [
 						"[]",
 						"int64"
@@ -2891,6 +2905,14 @@
 					"Typewords": [
 						"ModSeq"
 					]
+				},
+				{
+					"Name": "MsgIDs",
+					"Docs": "Message.ID, for erasing, order does not necessarily correspond with UIDs!",
+					"Typewords": [
+						"[]",
+						"int64"
+					]
 				}
 			]
 		},
@@ -3214,13 +3236,13 @@
 	],
 	"Ints": [
 		{
-			"Name": "UID",
-			"Docs": "IMAP UID.",
+			"Name": "ModSeq",
+			"Docs": "ModSeq represents a modseq as stored in the database. ModSeq 0 in the\ndatabase is sent to the client as 1, because modseq 0 is special in IMAP.\nModSeq coming from the client are of type int64.",
 			"Values": null
 		},
 		{
-			"Name": "ModSeq",
-			"Docs": "ModSeq represents a modseq as stored in the database. ModSeq 0 in the\ndatabase is sent to the client as 1, because modseq 0 is special in IMAP.\nModSeq coming from the client are of type int64.",
+			"Name": "UID",
+			"Docs": "IMAP UID.",
 			"Values": null
 		},
 		{