Refactor get_failover_log/1

Create a get_failover_log/2 that can be called without a gen_server
call in case the state is already available.

This change is needed for a subsequent commit where you need to get
the failover log within a gen_server:handle_call/3.

Change-Id: I0f5b5f11d1b63eb9f0fc2b21d1cef66a5706ac5f
Reviewed-on: http://review.couchbase.org/34433
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>