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.
for a uid set, the syntax <num>:* must be interpreted as <num>:<maxuid>. a
wrong check turned the uid set into <maxuid>:<maxuid>. that check was meant for
the case where <num> is higher than <maxuid>, in which case num must be
replaced with maxuid.
this affected "uid expunge" with a uid set, possibly causing messages marked
for deletion not to be actually removed, and this affected "search" with the
uid parameter, possibly not returning all messages that were searched for.
found while writing tests for upcoming condstore/qresync extensions.
