Spin-off from SERVER-21560

1. Start with a 3-member replica set, with all members reflecting the same point in time.
2. Restart one of the secondaries.
3. The mongoD log will contain the following in regards to sync-source choice:

   2015-11-19T11:25:06.306-0500 I REPL     [ReplicationExecutor] could not find member to sync from
   

4. No further sync source messages will be written until a write is handled by the primary.

The current message "could not find member to sync from" could be confused to be an error condition. We should instead write something like "X potential sync sources found, waiting for write activity to choose a source"