[ConnectionPool]: Add feature to run closure on all open connections

[adam-fowler]

Please summarize your feature request

Provide access to all the open connections in the connection pool

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 all connections in a connection pool between read-only and writable. This is done by sending either the READONLY or READWRITE commands to the connection. To do this we need access to all the currently open connections.

Have you considered any alternatives?

#581