Add dns_cache for server addresses as in pgbouncer (#249)

* Add dns_cache so server addresses are cached and invalidated when DNS changes.

Adds a module to deal with dns_cache feature. It's
main struct is CachedResolver, which is a simple thread safe
hostname <-> Ips cache with the ability to refresh resolutions
every `dns_max_ttl` seconds. This way, a client can check whether its
ip address has changed.

* Allow reloading dns cached

* Add documentation for dns_cached

src/lib.rs

@@ -1,6 +1,7 @@
 pub mod auth_passthrough;
 pub mod config;
 pub mod constants;
+pub mod dns_cache;
 pub mod errors;
 pub mod messages;
 pub mod mirrors;