Atomically set cluster.xml and stores.xml in metadata store
Fix for problems on metadata change with less nodes

1. Check for node existence in InvalidMetadataCheckingStore and
throw InvalidMetadataException if specified node is not in the cluster
2. Have ZenStoreClient rebootstrap when getting InvalidMetadataException.
This fixes if the new cluster does not have a node that ZenStoreClient query
data from, it would throw Exception.
Add Zone Shrinkage End To End Test
Fix set-metadata behavior on store metadata version; added test
Add to EndToEndZoneShrinkageTest

1. Fixed ZenStoreClient NPE related to uninitialized sysStore
2. Added test to verify:
  a. Slops does stream out correctly from dropping servers
  b. Slops targeting dropping servers get dropped
3. ZenStoreClient pickup the store and cluster changes