[WIP] Allow disabling clustering for development environments

[strugee]

In particular, debugging with node debug is difficult when clustering is enabled.

This is a WIP, not sure when I'll have time to finish it. Call this PR a note to self to finish up this branch.

[ghost]

In Node.js >= 6 with node --inspect and children == 1 works fine, but with --inspect-brk or node inspect bin/pump the process die and the logs shows

{"name":"pump.io","hostname":"parabola","pid":22381,"component":"cluster","level":50,"msg":"worker 22458 died (12). restarting...","time":"2018-03-21T17:28:01.641Z","v":0}
Debugger listening on ws://127.0.0.1:9241/50dba07c-2492-47c0-9026-f3a98d7a0b94