mds: obviate MDLog::start_entry

mds: obviate MDLog::start_entry

The major problem here is that the MDLog::_start_entry method puts the
current event sequence number in the EMetaBlob of the event (if
present). Because of this, no other event can be submitted as this would
invalidate the event sequence. Instead, fixup the event sequence during
submission and simplify related logic that uses it during EMetaBlob
construction.

Secondarily, for the purposes of this commit series, _start_entry
introduced recursive locks when generating the ESubtreeMap within
MDLog::_segment_upkeep. So, this commit is a necessary cleanup.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>