tbc: fix race condition in fork tests

[AL-CT]

Summary
Fixes a race condition that occurs due to multiple blockheader messages being sent to TBC before it has time to read them.

Changes

[marcopeereboom]

Let's fix this for real. Why is the write stepping on each other?

We need a mutex somewhere.

[marcopeereboom]

This PR adheres to the readers read, writers write idiom. I recall reasoning through that when I wrote it and argued the long mutex away in writes. That was a mistake on my end. Apologies to the poor CI that suffered through this.