Add a simple thread pool

[neo1973]

Description

Implement a simple thread pool so threads can be reused instead of recreated.

Motivation and context

inputstream.adaptive creates a lot of threads though std::async or by instantiating std::thread directly. E.g. watching this stream I measured about 90 threads being created and destroyed per second (very inaccurate measure using a stopwatch and GDB debug output):

#KODIPROP:mimetype=application/dash+xml
#KODIPROP:inputstream=inputstream.adaptive
#KODIPROP:inputstream.adaptive.manifest_type=mpd
#A new period every 3 min (20 times/hour)
#no segment updates needs inserting
https://livesim.dashif.org/livesim/periods_20/testpic_2s/Manifest.mpd

Creating threads is a non trivial task and reusing threads instead of recreating them improves efficiency.

How has this been tested?

Tested with the above stream and https://github.com/anxdpanic/plugin.video.youtube/. I confirmed in GDB on Linux that the thread pool grows to a size of only two and then no new threads are created.

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• I have read the Contributing document
• My code follows the Code Guidelines of this project
• My change requires a change to the Wiki documentation
• I have updated the documentation accordingly

[CastagnaIT]

interesting alternative solution, although the ReadSampleAsync "functionality" should be completely rewritten in a future can in the meantime be an improvement
i will take a look on next days

[CastagnaIT]

please add appropriate namespaces such as the others utils files
at least the "utils"

namespace UTILS
{

[CastagnaIT]

tested on windows and android
not found problems