stream: RST no longer acks all data

stream: RST no longer acks all data

Since forever (1578ef1e3e8a24d0cc615430c4e6bec1fefdad28) a valid RST
would update the internal `last_ack` representation to include all
unack'd data. This was originally done to make sure the unACK'd data was
inspected/processed at flow timeout.

It was observed however, that if GAPs existed in this unACK'd data, a
GAP could be reported in the stats and a GAP event would be raised. This
doesn't make sense, as missing segments in the unACK'd part of the
stream are completely normal. Segments simply do not all arrive in
order.

It turns out that the original behavior of updating `last_ack` to
include all unACK'd data is no longer needed.

For raw stream inspection, the detection engine will already include the
unACK'd data on flow end.

For app-layer updates the unACK'd data is often harmful, as the data
often has GAPs. Parser like the http parser would report these GAPs and
could also get confused about the post-GAP data being a new transaction
including a file. This lead to many reported errors and fantom txs and
files.

Since the GAP detection uses `last_ack` to determine GAPs, not moving
`last_ack` addresses the GAP false positives.

Ticket: #7422.