Don't cancel allocation when a new sync id is found on shared filesystems

[dakrone]

In ReplicaShardAllocator.processExistingRecoveries, if we find a "better" match, we cancel allocation of a replica:

// we found a better match that has a full sync id match, the existing allocation is not fully synced
// so we found a better one, cancel this one
it.moveToUnassigned(new UnassignedInfo(UnassignedInfo.Reason.REALLOCATED_REPLICA,
        "existing allocation of replica to [" + currentNode + "] cancelled, sync id match found on node [" + nodeWithHighestMatch + "]"));

However, when on a shared filesystem, all data nodes have the same data, so we should not cancel allocation if a new node pops up.

[dakrone]

@bleskes I spoke with @brwe about this and I think we agreed it was worth doing, but I'm curious about your input on this as well.

[dakrone]

Shadow replicas have been removed and this is no longer applicable