Add lockfile per namespace

Attempt to create a lock file per namespace on initialization before
creating spool file directories. Error out if there is already a lock
file in place (which means the current process is a duplicate)

Update tests to reflect this.

Also updated existing tests to call marquise_shutdown, where applicable,
to cleanup locks from tests.

Addresses issue #23 
Again, need to handle the case where `unlink(2)` fails; at the moment, if the deletion fails marquise_shutdown returns as if everything succeeded.