lib/tdb: Add/expose lock functions to support CTDB

lib/tdb: Add/expose lock functions to support CTDB

This patch adds two lock functions used by CTDB to perform asynchronous
locking. These functions do not actually perform any fcntl operations,
but only increment internal counters.

 - tdb_transaction_write_lock_mark()
 - tdb_transaction_write_lock_unmark()

It also exposes two internal functions
 - tdb_lock_nonblock()
 - tdb_unlock()

These functions are NOT exposed in include/tdb.h to prevent any further
uses of these functions. If you ever need to use these functions, consider
using tdb2.

Signed-off-by: Amitay Isaacs <amitay@gmail.com>