child thread - segfaults on mac

Just the API and a bare-bone implementation

Requires synchronous and single-threaded, so only 1 channel and 1 outstanding RPC
per channel for now. No open-loop for now

When building the native extension make -j can absorb all available
resources on a system.  Implement "reasonable" limits on the number
of compiling jobs when the number of processors can be detected and
use a conservative count when ruby version does not provide detection.