IMAP: save updates to MODSEQ, HIGHESTMOSEQ and RECENT whenever they change

The changes to MODSEQ seem too costly at the first time, but the thing is that
these would only happen when the MODSEQ gets modified, which means that it's
very likely that something else is changing anyway -- e.g. the FLAGS of some
message. These writes are furthermore wrapped in a transaction which
encapsulates more of the writes together, and it has never been a problem
before. I don't suppose there'll be any performance problems triggered by this.

REVIEW: 113891