Batch partition loading from the metastore

OK here is the initial implementation that does batching based on time (a new parameter ``hive.metastore-partition-load-batch-interval`` is introduced) to address #1981 . Currently each hive-metastore thread does batching independently (instead of using locking and letting one thread do the batching as the same ``BackgroundCacheLoader`` is shared by all metastore threads). For keeping the state this implementation uses thread local variables to get rid of synchronization & make the code more concise. Once the code is ready to merge I will update the docs with the new parameter.
Each call to reload would return the `ListenableFuture` for the corresponding request, but it doesn't need to immediately schedule a request to `load`. It can accumulate the request and then later invoke a single `loadAll`