CASSANDRA-20291 Filter unreachable private IPs from batch log writing

[manmagic3]

Add filtering logic to remove reachable private IPs from list of nodes to be selected for batch log writes. Patch By Manish Khandelwal, Reviewed by TBD for CASSANDRA-20291

This patch handles batch log writing in case of loss of private IP connectivity with multi interface enabled ( public ip for multi data center). This logic is

1. Check for valid candiates for batch log writing using following logic

• Skip local node  (already in place)
  

• Skip if failure detector shows node down ( already in place)
  

• Check for intra dc connectivity on private network. Cache the result for 30 seconds (hard coded as of now can be made configurable). Valid for topology where two network interfaces are introduced (This logic is freshly introduced.)
  

[manmagic3]

@tommystendahl fixed your review comments

[tommystendahl]

LGTM.