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 
Why ten bytes? Seems like you might be able to get the appropriate value here from `sizeof(pid_t)`, and the number of bytes in the decimal string representation you can get with `snprintf`.