Use the lock also when reading, in AdapterRegistry

[olleolleolle]

Description

Adapter Registry stores information in "a thread-safe way" (using a synchronize block). But, when reading, it doesn't use the lock.

This PR tries to repair the issue reported in #1068, by using a synchronize block around the reading, too.

Todos

Remaining things

• Tests
• Document this kind of thing in the DEVELOP guide?
• Perhaps have a concurrency label? Is that the right word?

Additional Notes

This was inspired by watching a video from RWC2019. Here it is: https://www.youtube.com/watch?v=qX_FRa43r4k

[olleolleolle]

@ioquatix Hi 👋 ! Cool, I've taken the first step, now I guess the rest of the road is still to go.

[ioquatix]

It looks okay to me but it's been a while.

[iMacTia]

The fix itself looks good.
Not sure there's much we can do on the tests side though