ISPN-263 Handle cluster partitions

* Fix remaining test failures
* Small documentation/javadoc fixes
* Move PartitionContext and PartitionHandlingStrategy to impl package.
* Fix configuration exception when partition handling is enabled in
  replicated mode.
* Use i18n messages for AvailabilityExceptions
* Rename CacheTopology.isDegradedMode back to isMissingData
* Don't allow partition to enter degraded mode if it's already unavailable.