Enable config file re-reading with SIGUSR2

[steve-chavez]

Continuing the work on #1289. Should also fix #1119.

Considerations:

• SIGUSR2 only re-reads the config, but doesn't reload the schema cache. This is because jwt-secret and other settings don't deal with the db. Reloading the cache on a jwt-secret update would be wasteful.
• From the above, if a db-schema changes, a SIGUSR2 + SIGUSR1 would be needed for a correct schema cache reload.
• If the config file path has changed, the reload will fail(and show an error) but postgrest will continue to work. The reloading will be done in a different thread. This is probably not an issue in production since services have an absolute path defined for the config.
• If the config file has an invalid config option(like invalid roleClaimKey) the reload will also fail and pgrst will continue to work. In this case the user would have to fix the config for the reload to be successful.

Only these settings will be re-read:

• db-schema
• db-anon-role
• server-proxy-uri
• jwt-secret
• secret-is-base64
• jwt-aud
• max-rows
• pre-request
• role-claim-key
• db-extra-search-path
• app.settings.*

These setting won't be re-read(would require a restart):

• db-uri
• db-pool
• db-pool-timeout
• server-host
• server-port
• server-unix-socket
• server-unix-socket-mode

[steve-chavez]

I think this is done!

I've added bash tests for app.settings.*, jwt-secret, db-schema reloading with SIGUSR2. Those are really the main use case for SIGUSR2. Left testing other config options reload as TODOs.