Avoid distributing new catalog snapshot again for the transaction being decoded.

Hi,

When doing some other related work, I noticed that when decoding the COMMIT
record via SnapBuildCommitTxn()-> SnapBuildDistributeNewCatalogSnapshot() we
will add a new snapshot to all transactions including the one being decoded(just committed one).

But since we've already done required modifications in the snapshot for the
current transaction being decoded(in SnapBuildCommitTxn()), so I think we can
avoid adding a snapshot for it again.

Here is the patch to improve this.
Thoughts ?

Best regards,
Hou zhijie