add condstore & qresync imap extensions

for conditional storing and quick resynchronisation (not sure if mail clients are actually using it that).

each message now has a "modseq". it is increased for each change. with
condstore, imap clients can request changes since a certain modseq. that
already allows quickly finding changes since a previous connection. condstore
also allows storing (e.g. setting new message flags) only when the modseq of a
message hasn't changed.

qresync should make it fast for clients to get a full list of changed messages
for a mailbox, including removals.

we now also keep basic metadata of messages that have been removed (expunged).
just enough (uid, modseq) to tell client that the messages have been removed.
this does mean we have to be careful when querying messages from the database.
we must now often filter the expunged messages out.

we also keep "createseq", the modseq when a message was created. this will be
useful for the jmap implementation.

imapclient/protocol.go

@@ -1,6 +1,7 @@
 package imapclient
 
 import (
+	"bufio"
 	"fmt"
 	"strings"
 )
@@ -134,6 +135,20 @@ func (c CodeCopyUID) CodeString() string {
 	return fmt.Sprintf("COPYUID %d %s %s", c.DestUIDValidity, str(c.From), str(c.To))
 }
 
+// For CONDSTORE.
+type CodeModified NumSet
+
+func (c CodeModified) CodeString() string {
+	return fmt.Sprintf("MODIFIED %s", NumSet(c).String())
+}
+
+// For CONDSTORE.
+type CodeHighestModSeq int64
+
+func (c CodeHighestModSeq) CodeString() string {
+	return fmt.Sprintf("HIGHESTMODSEQ %d", c)
+}
+
 // RespText represents a response line minus the leading tag.
 type RespText struct {
 	Code    string  // The first word between [] after the status.
@@ -201,6 +216,12 @@ type UntaggedFetch struct {
 	Attrs []FetchAttr
 }
 type UntaggedSearch []uint32
+
+// ../rfc/7162:1101
+type UntaggedSearchModSeq struct {
+	Nums   []uint32
+	ModSeq int64
+}
 type UntaggedStatus struct {
 	Mailbox string
 	Attrs   map[string]int64 // Upper case status attributes. ../rfc/9051:7059
@@ -224,9 +245,16 @@ type UntaggedEsearch struct {
 	Max        uint32
 	All        NumSet
 	Count      *uint32
+	ModSeq     int64
 	Exts       []EsearchDataExt
 }
 
+// UntaggedVanished is used in QRESYNC to send UIDs that have been removed.
+type UntaggedVanished struct {
+	Earlier bool
+	UIDs    NumSet
+}
+
 // ../rfc/2971:184
 
 type UntaggedID map[string]string
@@ -278,6 +306,13 @@ func (ns NumSet) String() string {
 	return r
 }
 
+func ParseNumSet(s string) (ns NumSet, rerr error) {
+	c := Conn{r: bufio.NewReader(strings.NewReader(s))}
+	defer c.recover(&rerr)
+	ns = c.xsequenceSet()
+	return
+}
+
 // NumRange is a single number or range.
 type NumRange struct {
 	First uint32  // 0 for "*".
@@ -450,3 +485,8 @@ func (f FetchBinarySize) Attr() string { return f.RespAttr }
 type FetchUID uint32
 
 func (f FetchUID) Attr() string { return "UID" }
+
+// "MODSEQ" fetch response.
+type FetchModSeq int64
+
+func (f FetchModSeq) Attr() string { return "MODSEQ" }