Re-claim memory in C library periodically

The lvm C library requires that you close/re-open for long
running processes to reclaim memory.

Signed-off-by: Tony Asleson <tasleson@redhat.com>
Serialize service processing.

There are a number of things that targetd is calling that cannot
be executed concurrently.  Make the service only handle one
request at a time.

Signed-off-by: Tony Asleson <tasleson@redhat.com>