Consistently use 'mono_time' rather than 'monotime'

[zugz]

This change is 

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[codecov]

Codecov Report

Merging #1109 into master will increase coverage by <.1%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1109     +/-   ##
========================================
+ Coverage    82.6%   82.7%   +<.1%     
========================================
  Files          80      80             
  Lines       14316   14316             
========================================
+ Hits        11828   11841     +13     
+ Misses       2488    2475     -13

Impacted Files	Coverage Δ
toxcore/mono_time.c	100% <100%> (ø)	⬆️
toxav/audio.c	65.7% <0%> (-4.4%)	⬇️
toxav/msi.c	66.2% <0%> (-2%)	⬇️
auto_tests/toxav_many_test.c	96.6% <0%> (-0.7%)	⬇️
toxav/toxav.c	69.4% <0%> (-0.5%)	⬇️
toxcore/onion_client.c	95% <0%> (-0.2%)	⬇️
toxcore/Messenger.c	85.9% <0%> (ø)	⬆️
toxcore/net_crypto.c	94.3% <0%> (+0.6%)	⬆️
toxcore/friend_connection.c	94.5% <0%> (+0.8%)	⬆️
toxcore/group.c	73.1% <0%> (+1.4%)	⬆️
... and 2 more

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[iphydf]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained