lib-index: Remove tracking of mail_index_modseq_sync.highest_modseq It's no longer necessary after the previous changes.

Removed all the "enum foo;" declarations. They didn't seem to be very standard. Old gcc versions warn about them and C++ code doesn't like them.

lib-index: Added mail_index_have_modseq_tracking() needed for previous commit.

Added ability to specify message's minimum modseq value. --HG-- branch : HEAD

Fixed crashes related to struct mail_index_map_modseq handling. --HG-- branch : HEAD

Keep modseqs as 1 until the first modseq ext intro record enables them. Performance should be better again when modseqs are disabled. --HG-- branch : HEAD

Modseqs are no longer calculated from transaction log sequence + offset. Now they begin from 1 and each "visible" transaction increases it by one. --HG-- branch : HEAD

Added QRESYNC support. --HG-- branch : HEAD

Left out from initial CONDSTORE commit. --HG-- branch : HEAD