Respond to JSONP send requests with the correct MIME type

#3110
Rewrite ProcessStartRequests to not use async/await to avoid deadlocks

- Deadlocks would occur in the old async/await code if someone attached
  to the PreSendRequestHeaders event.

#3116
Attach to the PreSendRequestHeaders event for only one test
Bump the ConnectionsDisconnected PC even when OnDisconnected throws
Removed the old OnDisonnected overload

- Removing the old overload is preferable to changing its behavior

#3115
Add a workaround to make sure Redis connection is reconnected in ConnectionRestored event

#3095
Merge branch 'release' into dev
DumpingDisconnect.Net

Removing handling Disconnect message in the .NET Client as the server no longer sends Disconnect messages.
DumpingDisconnect.JS

Removing handling Disconnect message in the .JS client as the server no longer sends Disconnect messages.