DBObjectMap: lock header_lock on sync()

DBObjectMap: lock header_lock on sync()

Otherwise, we can race with another thread updating state.seq
resulting in the old, smaller value getting persisted.  If there
is a crash at that time, we will reuse a sequence number, resulting
in an inconsistent node tree and bug #9891.

Fixes: 9891
Backport: giant, firefly, dumpling
Signed-off-by: Samuel Just <sjust@redhat.com>
(cherry picked from commit 2b63dd25fc1c73fa42e52e9ea4ab5a45dd9422a0)

Conflicts:
	src/os/DBObjectMap.cc
        because we have state.v = 1; instead of state.v = 2;