Avoid contention in updateLinksTimestamp()

Avoid contention in updateLinksTimestamp()

Since LinksUpdate::doUpdate() already flushes the transaction,
go ahead and flush before other DataUpdates might run (e.g.
from RefreshLinksJob). Also release the lock before running
the LinksUpdateComplete handlers, as the lock is just to keep
LinksUpdate instances from racing with each other.

Change-Id: Ied97fa36fbca0203123e9fc966d2e23bfd621c0e