If the reconstructor finds a fragment that appears to be stale then it
will now quarantine the fragment. Fragments are considered stale if
insufficient fragments at the same timestamp can be found to rebuild
missing fragments, and the number found is less than or equal to a new
reconstructor 'quarantine_threshold' config option.
Before quarantining a fragment the reconstructor will attempt to fetch
fragments from handoff nodes in addition to the usual primary nodes.
The handoff requests are limited by a new 'request_node_count'
config option.
'quarantine_threshold' defaults to zero i.e. no fragments will be
quarantined. 'request node count' defaults to '2 * replicas'.
Closes-Bug: 1655608
Change-Id: I08e1200291833dea3deba32cdb364baa99dc2816
46ea3aeae8