Process Manager is now lib/process-manager.js
It's been entirely rewritten to reflect what I think a process manager
API should look like.
In particular, there are now two Process Managers, QueryProcessManager
and StreamProcessManager.
Pass QueryProcessManager a pure-ish query function (sync or async) that
takes a JSON value and returns a JSON value, and PM.query() will
execute that function in a subprocess, and return a Promise for its
return value.
StreamProcessManager is the same idea: Pass it a function to create an
ObjectReadWriteStream, and PM.createStream() will create a stream in a
subprocess and return a stream connected to it.
