Hub invocation callbacks are not cleaned up in JS client if no response is received causing a memory leak

[DamianEdwards]

Note: This is the 2.0 version of #2270

There's a memory leak when the hub proxy never receives a valid response from the server as the callback isn't cleaned up from the global callbacks map. Suggested changes:

• Hub invocation callbacks should be stored on the connection instance metadata member (connection._). Note it needs to be separate to the existing callback map on there used to store hub proxy client callbacks
• For the transports that use Ajax sends, the fail delegate should clear the callback delegate
• For WebSockets, the callback map for that connection should be completely cleared on the reconnect event
• For all transports, the callback map for that connection should be completely cleared on the disconnect event

[DamianEdwards]

Fixed in 42c7575

[gustavo-armenta]

tested behavior is the same as in #2270