Hive Metastore should send single AlterPartitionEvent with list of partitions

In HIVE-3938, work was done to send single AddPartitionEvent for APIs that add partitions in bulk. Similarly, we have alter_partitions APIs that alter partitions in bulk via a single HMS call. For such events, we should also send a single AlterPartitionEvent with a list of partitions in it.
This would be way more efficient than having to send and process them individually.

This fix will be incompatible with the older clients that expect single partition.