In getCache()/getQuarantineCache(), they return a reference to variable
guarded by a mutex. After llvm#67776, thread-safey analysis checks if a
variable return by reference has the lock held. The ASSERT_CAPABILITY
only claims after calling that function, the lock will be held. But not
asserting that the lock is held *before* calling that function.

In the patch, we switch to use REQUIRES() and assertLocked() to mark the
code paths. Also remove the misused ASSERT_CAPABILITY.