[ConnectionPool]: Provide method to close all the connections in a connection pool

[adam-fowler]

Please summarize your feature request

Provide method to close all the connections in a connection pool, so all connection states can be reset.

Describe the functionality you're seeking in more detail

This is a request for valkey-swift.

There are situations where we need to change the state of a connection in valkey-swift. The easiest way to do this is to do close all currently open connections and set state when they re-open.

Have you considered any alternatives?

#580