RemoteTranscoderManager should stick with transcoder as long as conditions are good

[kyriediculous]

Is your feature request related to a problem? Please describe.
This issue pertains to running an O/T split setup with remote GPU transcoders.

Currently for each segment from an incoming stream, the RemoteTranscodeManager will pick a new transcoder each segment. This is not ideal as it will result in GPU session startup overhead each time a transcoder gets re-selected after a few segments.

Under ideal circumstances the stream is transcoded by the same remote transcoder until there's a negative outcome: error, timeout, invalid result, ... .

Describe the solution you'd like
Add a Sessions concept to the RemoteTranscoderManager. A Session keeps track of the current Transcoder to be used for each ongoing stream ManifestID.

This can be tracked in a mapping map[manifestID]*remoteTranscoder.

When a remoteTranscoder is assigned a ManifestID its load will increase by 1.
When a remoteTranscoder is removed for a ManifestID (either due to bad performance, error or the segment loop for the manifestID timing out) its load will be reduced by 1.

If there is an existing session whenever a Transcode request for a segment comes in we will use the remoteTranscoder currently in the map rather than call selectTranscoder() for each incoming request.

Other than trying to improve O/T performance. This new per-stream rather than per-segment behaviour also opens the door for more suitable selection algorithms such as latency based selection.

The selectTranscoder and completeTranscoders functions can be re-used to manage sessions instead.
The biggest change/addition would be a way for the RemoteTranscoderManager to listen for segment loop closures (or run its own cleanup loop).