Add timeout parameter to addprocs function

[MilesCranmer]

Currently, the worker timeout for addprocs can only be configured via the JULIA_WORKER_TIMEOUT environment variable. It would be more convenient to have a timeout keyword argument directly in the addprocs function.

Proposed API:

addprocs(4; timeout=120.0)  # 120 second timeout

This would be more ergonomic than having to set environment variables before calling addprocs.

[ViralBShah]

Probably should be filed in https://github.com/JuliaLang/Distributed.jl

[MilesCranmer]

Ok; can you transfer this over there? I don't have permissions to move issues