In addition to read-lock and write-lock, add a "priority access"
mechanism. The goal is to give priority to UI requests.

Several callers may acquire a "priority access". As soon as at least
one caller has priority access, all non-priority callers are blocked on
lock_read() and lock_write(), until all priority accesses are released.
However, they can continue executing their current requests following
the traditional RW-lock rules.

Acquiring a priority access is "immediate". It is different from a lock
in that it never blocks but makes other clients wait.