update with internal repo
made this library lua-releng clean.
added .gitignore.
feature: now we keep track of the key being fetched upon the last entry of the critical section.
removed a redundant require() statement.
replace the unreliable "last_key" field in the critical_sections record with a "workers" hash table which keeps track of all the active shcache backend query processes.
abort the connection directly when the client aborts the connection prematurely and there is no active critical sections.
debug: added more details to the error message "failed to acquire the lock".
removed the Lua traceback for the error "failed to acquire the lock" because it is too noisy.
Accept TTL from external_lookup callback which overrides positive_ttl