Rearrange encoder threading.

Rearrange encoder threading.

Soon we'll add a new encoder type, and the existing structure was
already creaking a bit at the seams while handling local and remote
encodes.  Here we split out an encoder thread and introduce the concept
of a "sync" thread (which blocks while the encoding is happening).
Later we'll have another type which submits the encode request to a
GPU and receives the reply back later.