Support multiple processes for CustomHandlers

cc @anthonychu

Rpc layer supports launching multiple worker processes: https://docs.microsoft.com/en-us/azure/azure-functions/functions-best-practices#use-multiple-worker-processes

Tracking item to extend same support for CustomHandlers

Start by reading the RPC layer's existing support for multiple worker processes and the linked Azure Functions best-practices guidance. Compare that behavior with CustomHandlers; done means CustomHandlers supports multiple worker processes equivalently, though the issue names no implementation files or tests.

Written by the indexing model from the issue text.