Add intermediate HubConnection states in Java client

[mikaelm12]

After looking at our locking and state transitions, we should add intermediate connection states Connecting and Disconnecting. This is for better handling potential races between starting and stopping a connection.
Context: #12078 (comment)

[mikaelm12]

Putting in preview 8 since this should help protect against potential races in the client.

[analogrelay]

Triage: We'll leave this in preview 8 but it's not a top priority. We have identified some races and knocked those down. Hardening this seems like a good 3.1 candidate.

[analogrelay]

Moving to backlog for 3.1