Only allow sending one blocking api command at a time.

[pragmatrix]

FreeSWITCH documentation states at https://developer.signalwire.com/freeswitch/FreeSWITCH-Explained/Modules/mod_event_socket_1048924/#31-api:

Send a FreeSWITCH API command, blocking mode. That is, the FreeSWITCH instance won't accept any new commands until the api command finished execution.

free-socks currently supports sending multiple blocking and - perhaps soon - other commands by letting clients clone the Client instance. Preventing the clone would not solve the problem, because async functions can be used as futures and initiated concurrently as long &self is used. An alternative would be to prevent cloning by using &mut self, schedule blocking commands, or detect the race condition and return an error?

And what about events, can they be received in between?