Clarified expectations about which threads are doing what. ApnsConnections now handle everything directly in IO threads; handlers are now responsible for spawning new threads when needed.
Revert "Documented the new listenerExecutorService param."

This reverts commit 759391d430635368c49379c332aaa402fc0ebefb.
Revert "Resolved a pair of TODOs by exposing the listener executor group to the ApnsConnection class."

This reverts commit b2d8a053df553e7ef2102e2fd3931879865644b7.
Merge branch 'threadless_connections' into connection_failure_listener
Wait for connection operations to finish in the push manager's listener executor service.