plugin: mysql-upstreams-api

[jonmorehouse]

Introducing an upstream plugin which exposes an API through gatekeeper allowing for registration of services and backends via HTTP.

Specifically, this does the following:

• [x] configures, starts and stops an api listening on a gatekeeper-user configurable port and registers it with gatekeeper itself to receive traffic at the upstreams prefix.
• [x] exposes the following endpoints:
  • add_upstream
  • add_backend
  • remove_backend
  • health
  • remove_upstream
  • info
• [x] auto-registers all previously persisted backends and upstreams upon startup (and periodically)

[jonmorehouse]

Oh yeah - fixed a bug in the plugin/upstream code base where we were calling Stop() twice on each upstream plugin.

Tests coming soon!!!

[jonmorehouse]

A few notes so I don't forget:

• Plugin should handle the database lifecycle (ie: calling Connect / Disconnect on it)
• Manager should accept configuration and needs to start a loop to periodically fetch upstream/backends from the DB.
• We'll need to make some index guarantees at the database level to ensure that we don't, for instance have duplicate names of upstreams.