Make NACK buffer cleanup on outgoing keyframe configurable (see #2401)

[lminiero]

See this post and #2401 for reference.

This PR adds a new janus.jcfg property called nack_optimizations to enable the behaviour master currently has, that is, the ability to clean the NACK buffer any time we're sending a keyframe to the peer. Quoting from the configuration file, which explains the property and the rationale:

# Janus can do some optimizations on the NACK queue, specifically when
# keyframes are involved. Namely, you can configure Janus so that any
# time a keyframe is sent to a user, the NACK buffer for that connection
# is emptied. This allows Janus to ignore NACK requests for packets
# sent shortly before the keyframe was sent, since it can be assumed
# that the keyframe will restore a complete working image for the user
# anyway (which is the main reason why video retransmissions are typically
# required). While this optimization is known to work fine in most cases,
# it can backfire in some edge cases, and so is disabled by default.
#nack_optimizations = true

The property can also be dynamically changed via Admin API, e.g.:

curl -d '{"janus": "set_nack_optimizations", "admin_secret": "janusoverlord", "transaction": "123", "nack_optimizations": true}' http://localhost:7088/admin

[atoppi]

lgtm 👍

[lminiero]

Merging.