Add Manual host banning to PgCat (#340)

Sometimes we want an admin to be able to ban a host for some time to route traffic away from that host for reasons like partial outages, replication lag, and scheduled maintenance.

We can achieve this today using a configuration update but a quicker approach is to send a control command to PgCat that bans the replica for some specified duration.

This command does not change the current banning rules like

Primaries cannot be banned
When all replicas are banned, all replicas are unbanned

src/errors.rs

@@ -6,6 +6,7 @@ pub enum Error {
     SocketError(String),
     ClientBadStartup,
     ProtocolSyncError(String),
+    BadQuery(String),
     ServerError,
     BadConfig,
     AllServersDown,