Skip getPartition() call to Hive Metastore when a partition already exists

Currently, we compute a diff between the current partition and an already registered partition when a partition has already been registered in Hive. This is done by calling getPartition() on the Hive metastore client, which can be expensive. Since no time-varying attributes are stored in a Hive partition, diff computation (and getPartition() call) can be skipped when a partition already exists.