This pr mainly has two changes:
1. add some merge processes about partition events
2. add a ut for `MetastoreEventFactory`. First add some mock classes (`MockCatalog`/`MockDatabase` ...) to simulate the real hms catalog/databases/tables/partitions, then create a event producer which can produce every kinds of `MetastoreEvent`s randomly. Use two catalogs for test, one is named `testCatalog` and the other is the `validateCatalog`, use event producer to produce many events and let `validateCatalog` to handle all of the events, but `testCatalog` just handles the events which have been merged by `MetastoreEventFactory`, check if the `validateCatalog` is equals to `testCatalog`.
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