Better sharing for Executors (thread pools)

[swankjesse]

Currently each SPDY connection creates its own ExecutorService for read, write and callbacks. We could do the same work with fewer resources by using fewer ExecutorServices.

Along with this we should improve thread naming, so that stack dumps show application-level thread information, like SPDY reader: http://api.squareup.com or SPDY callback: http://api.squareup.com stream 5.