Improve ShardedDOMDataTreeProducer locking

The path from backend was taking coarse lock even when not needed.
Explicitly annotate submitTransaction() as needing the lock being
held and push down its acquisition so we take it only after we
are certain we actually need it.

Change-Id: Icd1226796568829ea3735e6eec42677a79b9b3b5
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit 2ce13c8680d68ba1e50eab9609069b6aecfa62a5)