Refactor stats to use atomics (#375)

* Refactor stats to use atomics

When we are dealing with a high number of connections, generated
stats cannot be consumed fast enough by the stats collector loop.
This makes the stats subsystem inconsistent and a log of
warning messages are thrown due to unregistered server/clients.

This change refactors the stats subsystem so it uses atomics:

- Now counters are handled using U64 atomics
- Event system is dropped and averages are calculated using a loop
  every 15 seconds.
- Now, instead of snapshots being generated ever second we keep track of servers/clients
  that have registered. Each pool/server/client has its own instance of the counter and
  makes changes directly, instead of adding an event that gets processed later.

* Manually mplement Hash/Eq in `config::Address` ignoring stats

* Add tests for client connection counters

* Allow connecting to dockerized dev pgcat from the host

* stats: Decrease cl_idle when idle socket disconnects

Cargo.toml

@@ -37,6 +37,7 @@ exitcode = "1.1.2"
 futures = "0.3"
 socket2 = { version = "0.4.7", features = ["all"] }
 nix = "0.26.2"
+atomic_enum = "0.2.0"
 
 [target.'cfg(not(target_env = "msvc"))'.dependencies]
 jemallocator = "0.5.0"