Support stateful protocols such as MySQL

At present, MetaProtcol Proxy only supports stateless protocols, primarily RPCs. However, I believe it would be beneficial to also expand support for stateful protocols, including MySQL and other DB protocols.

To achieve this, we will need to:

* Add a flag to application protocols to indicate whether they are stateless or stateful.
* If a protocol is stateful, routing will be based on connection level instead of request level.
* For "server speak first" protocols like MySql, establish an upstream connection once downstream connection is established.
* Most of the layer-7 filters, such as local/global rate limiting and auth, should be able to be reused with stateful protocols. However, since routing is performed at connection level, router will not be used.

@aeraki-mesh/metaprotocol-proxy-maintainers