Replace sleep()/usleep() with std::chrono calls

[ferdnyc]

This PR eliminates all remaining calls to sleep() and usleep() in the library (except in the Decklink files; I don't mess with those). In both C and C++, sleep() and usleep() are considered obsolete, and will be flagged as a potential security issue by many static analysis tools, due to their poor interaction with things like SIGALRM.

In C++11, the std::this_thread::sleep_for() STL class provides a feature-rich replacement.

(This is only a stopgap measure, as it would be better to eliminate these sleeps entirely. Sleeping is generally inadvisable — it's better to use timers or waitables. Here's Jules, the JUCE developer railing against the use of sleep() to delay program execution.)

[codecov-io]

Codecov Report

Merging #473 into develop will increase coverage by 0.43%.
The diff coverage is 4.00%.

@@             Coverage Diff             @@
##           develop     #473      +/-   ##
===========================================
+ Coverage    48.33%   48.76%   +0.43%     
===========================================
  Files          128      129       +1     
  Lines         9962    10037      +75     
===========================================
+ Hits          4815     4895      +80     
+ Misses        5147     5142       -5     

Impacted Files	Coverage Δ
src/Frame.cpp	47.70% <0.00%> (+1.16%)	⬆️
src/Qt/AudioPlaybackThread.cpp	0.00% <0.00%> (ø)
src/Qt/PlayerPrivate.cpp	0.00% <0.00%> (ø)
src/Qt/VideoCacheThread.cpp	0.00% <0.00%> (ø)
src/FFmpegReader.cpp	68.79% <50.00%> (ø)
src/ZmqLogger.cpp	29.33% <100.00%> (ø)
src/FFmpegWriter.cpp	62.48% <0.00%> (-0.60%)	⬇️
include/DummyReader.h	0.00% <0.00%> (ø)
tests/DummyReader_Tests.cpp	100.00% <0.00%> (ø)
... and 1 more

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[ferdnyc]

I'm going to go ahead and merge this. It's been sitting here a while, and these usleep() calls are the only remaining security-related vulnerability still flagged in our Codacy report. Be nice to get an all-clear there.

[ferdnyc]

ARRGGH! Of course, now we still have 2 security flags left, because there are still those two usleep() calls in the DeckLink code I didn't want to touch.