cefsrc crash repro

Crash happens when you have two pipelines with cefsrcs:

• start pipeline #1
• stop #1 either manually or via the new js-signals feature
• start #2
• observe segfault in CefInitialize call

This happens both on master (57f3ec1) and on the sha prior to the merge of the signals fork (002f660) - where we just use sleeps to emulate the behaviour of the signals.

The repo has two binaries:

• cargo run --bin signals repros the issue using the signals (this will only work on 57f3ec1)
• cargo run --bin sleep repros the issue on both master and 002f660