update modseq when changing mailbox/server metadata, and also for specialuse changes, and keep track of modseq for mailboxes

i added the metadata extension to the imapserver recently. then i wondered how
a client would efficiently find changed metadata. turns out the qresync rfc
mentions that metadata changes should set a new modseq on the mailbox.
shouldn't be hard, except that we were not explicitly keeping track of modseqs
per mailbox. we only kept them for messages, and we were just looking up the
latest message modseq when we needed the modseq (we keep db entries for
expunged messages, so this worked out fine). that approach isn't enough
anymore. so know we keep track of modseq & createseq for mailboxes, just as for
messages. and we also track modseq/createseq for annotations. there's a good
chance jmap is going to need it.

this also adds consistency checks for modseq/createseq on mailboxes and
annotations to the account storage. it helped spot cases i missed where the
values need to be updated.

store/state.go

@@ -61,12 +61,14 @@ type ChangeThread struct {
 type ChangeRemoveMailbox struct {
 	MailboxID int64
 	Name      string
+	ModSeq    ModSeq
 }
 
 // ChangeAddMailbox is sent for a newly created mailbox.
 type ChangeAddMailbox struct {
 	Mailbox Mailbox
 	Flags   []string // For flags like \Subscribed.
+	ModSeq  ModSeq
 }
 
 // ChangeRenameMailbox is sent for a rename mailbox.
@@ -75,6 +77,7 @@ type ChangeRenameMailbox struct {
 	OldName   string
 	NewName   string
 	Flags     []string
+	ModSeq    ModSeq
 }
 
 // ChangeAddSubscription is sent for an added subscription to a mailbox.
@@ -95,6 +98,7 @@ type ChangeMailboxSpecialUse struct {
 	MailboxID   int64
 	MailboxName string
 	SpecialUse  SpecialUse
+	ModSeq      ModSeq
 }
 
 // ChangeMailboxKeywords is sent when keywords are changed for a mailbox. For
@@ -111,6 +115,7 @@ type ChangeAnnotation struct {
 	MailboxID   int64  // Can be zero, meaning global (per-account) annotation.
 	MailboxName string // Empty for global (per-account) annotation.
 	Key         string // Also called "entry name", e.g. "/private/comment".
+	ModSeq      ModSeq
 }
 
 var switchboardBusy atomic.Bool