Avoid use of global mutable state in mono_time on win32.

[iphydf]

This uses a trick to get read-write access to this from a const
member function, similar to C++ mutable, but uglier.

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This change is 

[codecov]

Codecov Report

Merging #1135 into master will decrease coverage by <.1%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1135     +/-   ##
========================================
- Coverage    82.8%   82.8%   -0.1%     
========================================
  Files          82      82             
  Lines       14473   14479      +6     
========================================
+ Hits        11991   11995      +4     
- Misses       2482    2484      +2

Impacted Files	Coverage Δ
toxav/video.c	67.4% <100%> (ø)	⬆️
toxcore/mono_time.c	92.8% <100%> (ø)	⬆️
toxcore/mono_time_test.cc	100% <100%> (ø)	⬆️
toxav/audio.c	65.7% <100%> (ø)	⬆️
toxcore/net_crypto.c	93.3% <100%> (-0.1%)	⬇️
auto_tests/dht_test.c	47.1% <0%> (-0.8%)	⬇️
toxcore/friend_connection.c	95.7% <0%> (-0.3%)	⬇️
auto_tests/conference_double_invite_test.c	100% <0%> (ø)	⬆️
auto_tests/lossy_packet_test.c	100% <0%> (ø)	⬆️
... and 8 more

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[zugz]

The mutable_this trick is quite ugly, though I don't see that it could cause
any actual problems.

Also it's a shame to have Mono_Time as an argument to the callback, since
user-defined callbacks can't do anything with it (it's an incomplete type).

So how about reverting to having current_time_monotonic_default be the
behaviour of current_time_monotonic when (current_time_callback == nullptr),
rather than being a callback itself.

Then if the const qualifier is dropped from the Mono_Time argument to
current_time_monotonic, the mutable_this field can be dropped.

[iphydf]

I've kept the callback argument, so user defined callbacks can call mono_time functions, e.g. to reset the callback to its default, or set it to something else, at the end of the current callback's execution. PTAL.

[nurupo]

Reviewed 10 of 10 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained