FILE_PING.findMembers() optimizations

Following on from JGRP-2288, a couple of possible optimizations/improvements to FILE_PING.findMembers() were identified.

1. After the initial call to readAll(), some corrective steps are taken if the local node address was not returned by readAll(). However, in the case where findMembers() is invoked by TP.fetchResponsesFromDiscoveryProtocol(), it is normal if the local node address is not returned, since the readAll() responses are filtered based on the members parameter.

To avoid unnecessary writes to the file or cloud store, it would be good to add some checks based on whether members is null or not. For example, the calls to write() and writeAll() should probably not occur unless members is null.

2. In the call to sendDiscoveryResponse(), the last parameter is always false. However, it seems possible for a coordinator to get to this point in some edge cases. Though I haven't been able to identify any clear bugs that this would lead to, it might be better to pass is_coord as the last parameter.