perf: Refactor onion_client.c do_friends() to reduce network traffic

[JFreegman]

This PR simplifies the logic determining how often we
send announce request packets to offline peers and how often
we attempt to re-populate our nodes lists. It also removes
some randomness from packet sending that was intended to
reduce network traffic but greatly increased the code complexity
and didn't really do all that much to help.

We now aggressively search the DHT for offline friends when we first
bootstrap, add a new friend, or when a friend goes offline, and
exponentially increase the lookup interval based on how many
successful lookup requests we've already made, with a max
interval of 60 minutes per node, per friend (with 8 max nodes).
Requests are also now spaced evenly apart for each node
which guarantees at least one lookup per friend, per n seconds,
where n depends on the current timeout.

We also no longer spam node requests whenever our list of living
nodes drops below the maximum. Instead, we repopulate the nodes
list only when it drops to or below half the maximum, or after
a 10 minute timeout.

I ran two identical tox instances simultaneously for 10+ hours using
the netprof branch, and found that these changes reduced total tox
network traffic by between 30% and 50% (there is some randomness
based on the amount of in-traffic you get). Results are below (click
links for full results):

Current master with UDP enabled:

Run time: 40341 seconds
Average kilobytes per second: 7.69
Average packets per second: 38
Average packet size: 201 bytes

Total bytes:            310048540
Total bytes sent:       186386371 (60.12%)
Total bytes recv:       123662169 (39.88%)

This branch with UDP enabled:

--- Tox network profile log dump ---
Run time: 40401 seconds
Average kilobytes per second: 3.90
Average packets per second: 27
Average packet size: 142 bytes

Total bytes:            157547766
Total bytes sent:       104530935 (66.35%)
Total bytes recv:       53016831 (33.65%)

Current master with UDP disabled:

Run time: 48829 seconds
Average kilobytes per second: 0.99
Average packets per second: 5
Average packet size: 184 bytes

Total bytes:            48251725
Total bytes sent:       37546683 (77.81%)
Total bytes recv:       10705042 (22.19%)

This branch with UDP disabled:

Run time: 48843 seconds
Average kilobytes per second: 0.47
Average packets per second: 3
Average packet size: 124 bytes

Total bytes:            23143619
Total bytes sent:       17819044 (76.99%)
Total bytes recv:       5324575 (23.01%)

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This change is 

[codecov]

Codecov Report

Merging #2078 (0dca481) into master (6c2014d) will increase coverage by 0.06%.
The diff coverage is 96.51%.

@@            Coverage Diff             @@
##           master    #2078      +/-   ##
==========================================
+ Coverage   78.15%   78.22%   +0.06%     
==========================================
  Files         117      117              
  Lines       21516    21527      +11     
==========================================
+ Hits        16816    16839      +23     
+ Misses       4700     4688      -12     

Impacted Files	Coverage Δ
toxcore/onion_client.c	91.25% <96.51%> (-0.63%)	⬇️
toxcore/onion_announce.c	90.41% <0.00%> (-0.92%)	⬇️
toxcore/net_crypto.c	89.43% <0.00%> (-0.75%)	⬇️
toxcore/friend_connection.c	83.06% <0.00%> (-0.70%)	⬇️
toxcore/onion.c	87.18% <0.00%> (-0.28%)	⬇️
toxcore/DHT.c	83.77% <0.00%> (ø)
toxcore/Messenger.c	83.83% <0.00%> (+0.14%)	⬆️
toxav/toxav.c	69.37% <0.00%> (+0.29%)	⬆️
toxav/groupav.c	75.91% <0.00%> (+1.33%)	⬆️
... and 2 more

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