mds: create only one ESubtreeMap during fs creation

Previously we would create an empty ESubtreeMap when we opened the log
segment and then immediately journal a second one that created the root
and mdsdir.  More importantly, for the second ESubtreeMap, we would not
wait for it to commit before requesting the ACTIVE state, leading to
#4894.

Instead, break start_new_segment() into two steps: one that creates the
in-memory LogSegment tracking structure, and one that journals the
ESubtreeMap.  Open things early and write the (one) ESubtreeMap at the
end of boot_create().. and then wait for it.

Fixes: #4894
Signed-off-by: Sage Weil <sage@inktank.com>