An Open packet would trigger the use of a worker pool, then the Stat
packet would come in go to the pool and return faster than the Open
(returning a file-not-found error). This fixes that be eliminating the
pool/non-pool state switching.

The include test doesn't really exercise it fully as it cannot inject
a delay in the right place to trigger the race. I plan on adding a means
to inject some logic into the packet handling in the future once I
rewrite the old filesystem server code as a request-server backend.

Fixes #265