imapserver: implement MULTIAPPEND extension, rfc 3502

MULTIAPPEND modifies the existing APPEND command to allow multiple messages. it
is somewhat more involved than a regular append of a single message since the
operation (of adding multiple messages) must be atomic. either all are added,
or none are.

we check as early as possible if the messages won't cause an over-quota error.

imapserver/server_test.go

@@ -333,6 +333,14 @@ func xparseNumSet(s string) imapclient.NumSet {
 	return ns
 }
 
+func xparseUIDRange(s string) imapclient.NumRange {
+	nr, err := imapclient.ParseUIDRange(s)
+	if err != nil {
+		panic(fmt.Sprintf("parsing uid range %s: %s", s, err))
+	}
+	return nr
+}
+
 var connCounter int64
 
 func start(t *testing.T) *testconn {