Cancel Syncs that aren't making progress.

BUG: 18266674
1) If a sync has up/downloaded less than 10bytes in 60 seconds it is
considered to be making no progress and is summarily cancelled.

2) Apply a 30min hard time-out to initialization syncs.

Note that there is little proof that cancelling a sync has an
impact. All it results in is a Thread.interrupt on the sync
thread, which the adapter must itself implement. To this effect
this CL also updates the javadoc to make this clearer, and adds
some (unimplemented) threats about killing the hosting process.

Change-Id: I83c447648152ccbf76bb1fbd7e9216e01a37952f