fix(get) lookup in shm after lock timeout
fix(get) lookup in shm after lock timeout

Under high load, a reported rare race condition could occur:

When the underlying shm is being locked by other workers, a subsequent
worker might see its lock (`resty.lock`) timing out. If this happens
however, all hope is not lost and we should still attempts to read from
the shm: maybe the 1st worker holding the lock had a chance to fetch the
data, and our current timeout was a tiny race condition window.

Thanks Julien Desgats from Cloudflare (@jdesgats) for the report and
original patch in #49.

thibaultcha/lua-resty-mlcache