Create a sleep mechanism for slowing down mining process

[James4Ever0]

This pull request provides a configurable environmental variable called XMRIG_SLEEP_NANOSECONDS which value is default to zero.

One can configure it to slow down the mining process, effectively eliminate the issue of utilizing a single thread 100% at all the time, thereby increase the overall system stability and lower the thermo pressure.

Fix #1322

[SChernykh]

This is a weird place to put sleep into. If anything, it should be in the main CPU mining loop, next to already existing yield (with the similar purpose): https://github.com/xmrig/xmrig/blob/master/src/backend/cpu/CpuWorker.cpp#L357

[James4Ever0]

Now I have made some progress here. I make the dataset preparation process single threaded in ASM mode (because I cannot figure out how to sleep in assembly code in PIE compilation mode), throttled in non-ASM mode but taking ages to prepare so therefore out of major concern, and finally throttled hash functions.

[SChernykh]

Honestly, this PR is a bunch of hacks on top of each other, and there is 0% probability it will be merged. It's fine for your own purposes, I guess.

[SChernykh]

https://xmrig.com/docs/miner/command-line-options
--randomx-init=N thread count to initialize RandomX dataset

[James4Ever0]

That being said, I think it is possible to create some better mechanism for controlling thread occupation, by first measuring the average time to perform a single loop in hash function, then sleep accordingly to the measured time, so that the user will only have to provide a value ranging from 1 to 99 in percentage, simplifying the guesswork.