hotfix(globalpatches) use OpenSSL to seed PRNG

[thibaultcha]

Note: this is a hotfix to be manually merged in both master and next

Summary

This changes the seeding technique for LuaJIT's PRNG from using a
combination of time (s precision) + worker PID to using OpenSSL's
RAND_bytes().

Reasoning: in modern deployment setups such as application containers
and AWS AMIs (etc...), it is a common practise to deploy from a forked
VM, resulting in high probability of collision for PIDs at a seconds
precision. This could result in duplicated PRNG seeds, which is
ultimately the PRNG used to generate UUIDs in Kong, by the use of
lua-resty-jit-uuid.

Solution: in order to have a higher entropy when seeding LuaJIT's PRNG,
a proposed fix was to use /dev/urandom. This implementation however
uses OpenSSL's RAND_bytes(), which has the advantage of returning an
error if the entropy is estimated to be too low. However, this won't
cover use cases where the VM has been forked, resulting in multiple VM
clones with a high entropy, but equal to that of the other clones. We
suggest that such deployment environment increase their cloned VMs
entropy before starting Kong.

Full changelog

• use OpenSSL's RAND_bytes() to read random bytes
• truncate the final seed to 12 digits to prevent integer overflows
• update fallback seeding technique (time + worker PID) to use ms
  precision, just in case
• introduce a new kong lua shared dict. This dictionary's purpose is
  to hold essential data through Kong's lifecycle, and should eventually
  only be used through safe_set() (an abstraction for this could be
  envisaged later on, but is not the purpose of this patch)
• chosen seeds for each worker are stored in the kong shm, and can be
  consulted via the / endpoint. There is currently no way to re-seed all
  the workers at once unless by sending SIGHUP, because only 1 worker
  would be receiving such a request through the Kong Admin API.
• update debug.traceback() calls to use lvl 2 of the call stack, to
  show the actual caller of our patched math.randomseed()
• update log messages to be more explicit

Issues resolved

Fix #1751 #1739 #1623

[thibaultcha]

Manually merged to both next and master.