Follow up on the patch adding transaction boundaries to xrow stream.

Follow up on the patch adding transaction boundaries to xrow stream.
Use tsn as an abbreviation for transaction identifier (transaction
sequence number). It is an important enough concept to use a short and
a convenient tag name for.
Deploy the name across the code - in names and comments.
Clarify comments.
Still use box_txn_id() as API method since box_tsn() and box_txn() would
be too easy to mistype.