certificate revocation

[zmstone]

we had a few customers asked for client certificate revocation support.

there are a few options for us to support it:

1. Make use of the Erlang's builtin crl check and cache functionality
   https://www.erlang.org/doc/man/ssl.html#type-crl_check
   https://www.erlang.org/doc/man/ssl.html#type-crl_cache_opts

2. Like 1, but implement the cache callbacks by ourselves.

3. Provide custom verify callback in SSL options, and implement ourown check and cache
   https://www.erlang.org/doc/man/ssl.html#type-custom_verify
   return {revoked, _} , in case the cert is revoked.

No matter how it is implemented, we should have a configuration and an HTTP API to interface the users.

• Bootstrap the cache from a file (path of which is configurable)
• Persist the list in database
• Add CLI/HTTP APIs to add to / delete from the revocation list (and cache)