comments elucidating why we check the transaction's first or last cursor against the scan limited cursor of the current page

do the same for the backwards pagination case

when a cursor is specified, bump scan_limit by one so we don't count it among the txs scanned. otherwise we'll be short one

There was a bug related to bounds for the first page, when it contains
the genesis checkpoint. This was fixed by refactoring `TxBounds` to
systematically use a half-open interval (inclusive lowerbounds and
exclusive upperbounds) when calculating and combining intervals to be
used on the DB, regardless of where the bounds came from and whether
they were inclusive or exclusive at source.

Also added a new test to exercise this behaviour -- all existing tests
also pass, unchanged.