Fix off-by-one in decoding causing one-record events to be skipped.

A ReorderBufferTransaction's end_lsn, the sentPtr advocated by
walsender keepalive messages, and the end location remembered by the
decoding get_*changes* SQL functions all use the location of the last
read record + 1. I.e. the LSN points to the beginning of the next
record. That cannot realistically be changed without changing the
replication protocol because that's how keepalive messages have worked
since 9.0.
The bug is that the logic inside the snapshot builder, which decides
whether a transaction's contents should be decoded, assumed the start
location would point towards the last byte of the last record. The
reason this didn't actually cause visible problems is that currently
that decision is only made for commit records. Since interesting
transactions always have at least one additional record - containing
actual data - we'd never skip a transaction.
But if there ever were transactions, or other events, with just one
record containing important information, we'd skip them after stopping
and restarting logical decoding.