Reset onion search rate for a friend when we see evidence that they are online

[zugz]

Combine last_reported_announced with last_seen in Onion_Friend.

This resets friend search rate in two circumstances where it should be
reset:

• when the friend gets a dhtpk packet to us;
• when a friend connection to the friend goes down.

---

This change is 

[codecov]

Codecov Report

Merging #1202 into master will increase coverage by 0.2%.
The diff coverage is 75%.

@@           Coverage Diff            @@
##           master   #1202     +/-   ##
========================================
+ Coverage    82.9%   83.1%   +0.2%     
========================================
  Files          82      82             
  Lines       14646   14645      -1     
========================================
+ Hits        12144   12176     +32     
+ Misses       2502    2469     -33

Impacted Files	Coverage Δ
toxcore/onion_client.c	96.1% <75%> (-0.1%)	⬇️
toxcore/friend_connection.c	93.6% <0%> (-0.9%)	⬇️
toxcore/DHT.c	76.6% <0%> (-0.1%)	⬇️
toxcore/LAN_discovery.c	85.8% <0%> (+0.9%)	⬆️
toxav/toxav.c	68.1% <0%> (+0.9%)	⬆️
toxcore/net_crypto.c	94.3% <0%> (+1.2%)	⬆️
toxav/msi.c	66.4% <0%> (+1.9%)	⬆️
toxav/audio.c	70.5% <0%> (+4.8%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b694dbc...d5d6e3f. Read the comment docs.

[iphydf]

Reviewed 1 of 1 files at r1.
Reviewable status: 0 of 1 approvals obtained

[iphydf]

@zugz can you move the PR title to the description (because it's an implementation detail) and make the title something that users reading the changelog will understand (something closer to what the current description says, i.e. the effect on the behaviour)?

[zugz]

Done