Fix and Improve TableDescriptor caching for bulk assignment

HBASE-6214 backported HBASE-5998 (Bulk assignment: regionserver optimization by using a temporary cache for table descriptors when receiving an open regions request), but it's buggy on 0.94 (0.96 appears correct):

    HTableDescriptor htd = null;
    if (htds == null) {
      htd = this.tableDescriptors.get(region.getTableName());
    } else {
      htd = htds.get(region.getTableNameAsString());
      if (htd == null) {
        htd = this.tableDescriptors.get(region.getTableName());
        htds.put(region.getRegionNameAsString(), htd);
      }
    }

i.e. we get the tableName from the map but write the regionName.

Even fixing this, it looks like there are areas for improvement:
1) FSTableDescriptors already has a cache (though it goes to the NameNode each time through to check we have the latest copy. May as well combine these two caches, might be a performance win as well since we don't need to write to multiple caches.
2) FSTableDescriptors makes two RPCs to the NameNode when it encounters a new table. So the total number of RPCs necessary for a bulk assign (without caching is):
#regions + #tables
(with caching):
min(#regions,#tables) + #tables = #tables + #tables = 2 * #tables

We can make this only one RPC, yielding:
#tables

Probably not a big deal for most users, but in a multi-tenant situation where the number of regions being bulk assigned approaches the number of tables being bulk assigned, this could be a nice performance win.

Benchmarks coming.