Replicator now runs in background, using iOS background-task API

Porting back the replicator backgrounding support from CBL (commit 89ec01d5b83571a2525cdd5d074338527b39ad87 on 8/15/2013) for those people not quite ready to leave TouchDb.

Instead of stopping when the app goes into the background, each active replicator now registers a background task with the OS and keeps running until it goes idle or the task runs out of time.