Add multi-core concurrent packet processing

[joseph-henry]

ZeroTier on multiple threads

This alpha-quality patch enables concurrent processing of packets in the RX and TX directions and appears to improve performance significantly in many situations (but also make some worse). It automatically scales thread count according to std::thread::hardware_concurrency() and can optionally be overridden at start time by setting the ZT_PACKET_PROCESSING_CONCURRENCY environment variable. You can enable CPU pinning at start time via ZT_CPU_PINNING=1.

This has only been tested on Linux and FreeBSD and is not fully implemented in macOS so don't even try.

Example usage (ZT_PACKET_PROCESSING_CONCURRENCY is entirely optional since it'll automatically use all cores):

ZT_CPU_PINNING=1 ZT_PACKET_PROCESSING_CONCURRENCY=4 ./zerotier-one

To force ZeroTier onto one thread:

ZT_PACKET_PROCESSING_CONCURRENCY=1 ./zerotier-one

[laduke]

Awesome!
Can you make it not compile on mac, windows, etc? I know you know it doesn't work there, but it's worth testing ifdefs.
I made myself a branch with all the current PRs, and this one makes that branch not work on my mac (obviously).
I'm not sure if it's feasible to make it local.conf setting, so we can get the code in, but not enabled by default, but that would be cool IMO.

[Sandros94]

I wonder if this could improve performance on smaller cpus like the ones in commercial NASs