Re: Historic snapshot doesn't track txns committed in BUILDING_SNAPSHOT state

According to the comment above DecodeTXNNeedSkip(), transactions committed before SNAPBUILD_CONSISTENT state won't be decoded. It's important for initial table data synchronization because the table sync workers use the consistent snapshot to copy data so transactions before SNAPBUILD_CONSISTENT are already included in the initial data.

This change may be redundant with SnapBuildXactNeedsSkip(), I add just for safe.