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/errors.rs

@@ -19,6 +19,7 @@ pub enum Error {
     ClientError(String),
     TlsError,
     StatementTimeout,
+    DNSCachedError(String),
     ShuttingDown,
     ParseBytesError(String),
     AuthError(String),