Distinguish between globally-blocking and thread-blocking async functions

[cpcallen]

In the process of implementing threads, async native functions changed from blocking the whole interpreter to blocking only their own thread.

Tthe latter behaviour is probably the desired one in almost all cases, but:

• It should be possible to implement async native functions that do not allow other threads to run in the mean time, and
• All existing async native functions should be reviewed to ensure that they are in the correct category.