Engine: implement timeout with Go instead of external program

[camerondurham]

Currently, the runner uses the Linux timeout (from the coreutils homebrew formula on macOS) command to execute commands with a timeout.

This is to improve execution by using multiple channels and select to implement the timeout in Go. See gobyexample/timeouts for an example.

Acceptance Criteria (in progress):

• run command with configurable X second timeout
  • look at other options to implement timeout in Go
  • explore if we can use select with exec.Command instead of the timeout external process
  • test functionality in unit tests