fix: de-duplicate shm references

[flrgh]

Store a permanent reference to the lock shm object instead of using the memo table with the lock keys. Reduces the size+churn on the memo table by 50%.

This is greatly optimizes the happy path (when the lock is explicitly unlock()-ed by the caller and not by gc) because no table operations are done during garbage collection in this case. This makes resty.lock much less susceptible to memo table fragmentation.

[flrgh]

Snagged the bench script from #1 (comment) and ran it on this. Results:

$ resty --http-include lock_include.conf perflock.lua
original: 7.4079999923706
new: 6.9720001220703

[flrgh]

Results with jit.off(ref_obj); jit.off(unref_obj) in both files:

$ resty --http-include lock_include.conf perflock.lua
original: 14.645999908447
new: 14.079999923706