nbd-server: Add locking around transaction log writes

nbd-server: Add locking around transaction log writes

Bugfix for the previous patch:
nbd-server uses multiple processes and within each process multiple threads.
Thus: Locking is needed, to ensure that the data in the transaction log
is not corrupted.

Solution: Use sem_open(), the simplest solution.

Alternatives:
- shm_open() + a shared pthread_mutex

- fcntl() for cross process locking, and a ptrace_mutex for intra-process
  locking.

Signed-off-by: Manfred Spraul <manfred.spraul@de.bosch.com>
Signed-off-by: Wouter Verhelst <w@uter.be>