Fix sockproc startup race conditions after running out of memory

[GUI]

If the auto_ssl shared dict ran out of memory, and then nginx were reloaded, then a race condition existed where multiple "sockproc" processes could try to be started at the same time.

While I think this situation would be unlikely to affect a production system in a negative way (since the race condition only occurs during nginx reloads and 1 of the sockproc processes should still succeed and allow things to work), it did lead to some errors being logged, which would intermittently cause our tests to fail (it would crop up on the test following our t/memory.t test, since that next test would be the first reload following our test to explicitly exhaust the memory).

This is fixed by using the auto_ssl_settings shared dict for storing the resty-lock details (the lock prevents multiple processes from being started at once). This smaller shared dict (introduced in #68) is used for storing bits of data that won't grow in size so we can better ensure the data will never be evicted from the cache. I'm now able to repeatedly run the test suite in a loop without hitting this edge case.

Note that we are still using the auto_ssl shared dict for storing resty-lock details for domain registrations, since the memory requirements for that may grow (since there's a lock per domain, it's dynamic in size). But that should be okay, because similar to the SSL certs stored in auto_ssl, we're okay with cache evictions for old data in those cases (along with warnings being logged).

Also possibly relevant is that currently resty-lock always uses add for the shared dict (so it will evict old data to make room if necessary), but there's a pull request for allowing use of safe_add: openresty/lua-resty-lock#6 While we should be okay by switching things to auto_ssl_settings (since we should never have enough stored data in there to need evicting old data), it's something to keep an eye on.

Related to:

• Store hook secret in shared dict, don't overwrite it on reload #68 (comment)
• Attempt to get length of local 'certs' (a nil value)  #48 (comment)