Refactor onion_client.c do_friends()

This commit 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, for a
short duration until we've sent a certain number of successful
lookup requests. Thereafter, we send one lookup request every 3
minutes for each node associated with the friend (typically
between 4 and 8), for each friend.

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.

The effects of these changes is a decrease in overall tox network
traffic by about 30-40% with UDP enabled, and a bit less than
that with UDP disabled. The network may also be more reliable
overall because everything is significantly more deterministic
and less random