Cleanup network release

Fixes a handful of issues.  PendingRequests were getting touched on
random threads, so moved to handler thread.  Release could get
confused by state changes, so made it depend only on state set
during the request (if it's in the pending list, remove it, if it's
on the request list, release it).

bug:23080486
Change-Id: I62d3e279a34f3f3eac122842faa2abc8cb52b54c