Remove stdio's implicit init and require client.connect universally

[atesgoral]

From discussion: #327 (comment)

To minimize user impact while avoiding a breaking change, proceeding in the following steps looks appropriate:

• Support only the addition of the client.connect method Add client-level connect for initialize handshake #327
• Show a deprecation warning if client.connect has not been called in advance
• Raise an error if client.connect has not been called in advance