libctdb: track lock for each ctdb_db, complain if they hold too long.

libctdb: track lock for each ctdb_db, complain if they hold too long.

In particular, this stops them grabbing two (with wrappers so we can
enhance this logic once we support threads), and warns them if they
re-enter ctdb_service() holding a lock (you are not supposed to block!).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>