Fix context cancellation race condition

[cretz]

What was changed

We lock context fields potentially used by multiple goroutines and we make sure context.cancel is only called once.

Why?

Before there were situations (run tests with -race) where context variables were accessed concurrently without mutexes which could cause context cancel to run twice unintentionally.

Checklist

1. Closes Coroutine/context cancellation race conditions #720

[Sushisource]

Can we add the race flag to UTs in CI? (if it's not already)

[cretz]

@Sushisource - this flag is already set in CI (CI uses make unit-test which uses the TEST_ARG that has that value set). These tests would have failed CI without the accompanying fix.