Add _transaction_commit_time column to the change store metadata columns.

[baiyangtx]

For this issue #971 , the query on the last snapshot of the change store could get the historical records of a specific key, but it is still hard to get the history of the records after a specific timestamp. If we add the _commit_time column which is the commit time of the transaction to change store metadata columns the query could filter by _commit_time and get the history of the records after some timestamp.

[baiyangtx]

@wangtaohz @zhoujinsong @Aireed CC

[zhoujinsong]

Adding a _commit_time field when querying ChangeStore can indeed make it more convenient for users to use and filter data.
How would you like to implement it?