Route Synchronizer is not multi-worker-safe

[Firefishy]

The route Synchronizer is not threadsafe and fails to consistently reload routes across threads.

Test case:

1. Run puma with many threads and a single worker (or low count): puma -t 16:16 -w 0
2. Add 2 basic mocks. Mock A and Mock B
3. Run apache bench against Mock A: ab -c 20 -n 200000 http://URL-FOR-MOCK-A
4. Toggle Mock B on and off a few times.

Expected:
Routes reload as expected.

Observed:
Routes do not load consistently across some app threads and app throws errors due to missing routes.

(PR brewing)

[Firefishy]

Testing with puma -t 2:2 -w 8 is also a good check for worker issues.

[Firefishy]

Cc: @alkesh

[Firefishy]

In limited testing, it seems to be ok when running in single worker mode: -w 0