Process AllocWriteIdEvent do not add txnId to tableWriteIds mapping in Catalog

Description:

txnId to tableWriteIds mapping in Catalog(i.e., org.apache.impala.catalog.Catalog#txnToWriteIds_) is not added in the following cases:

1. When CREATE_TABLE event is followed by ALLOC_WRITE_ID_EVENT for a table in the same batch of MetastoreEventsProcessor.processEvents(), process AllocWriteIdEvent do not add txnId to tableWriteIds mapping in catalog. It happens, because we get table from catalog in the AllocWriteIdEvent constructor. And catalog_.getTable() returns null as the CREATE_TABLE is not processed by the time of AllocWriteIdEvent construction.Following events get the catalog table in the constructors:-
   1. AllocWriteIdEvent,
   2. ReloadEvent,
   3. CommitCompactionEvent.
2. When the table is not loaded, msTbl_ is null. process AllocWriteIdEvent returns without updating txnId to tableWriteIds map in catalog.