Fix several lock, async, exception-handling and counting bugs (+ tests)

[awolverp]

Fixed several bugs related to miss counting, missing await, lock cleanup on exceptions, and cancellation handling.

What was fixed

• Miss counter when lock is disabled
  With lock=False, misses were never incremented and hits were double-counted. Now they are counted correctly.

• Missing await on the ignore path
  In the async wrapper, when cachebox__ignore=True and lock was off, the function was not awaited and a coroutine object was returned. Now it is properly awaited.

• Lock leak when the function raises
  If the cached function raised an exception, the per-key lock was never removed and stayed in memory. It is now cleaned up properly.

• Waiters not decremented on cancellation
  If a task was cancelled while waiting for the lock, the waiter count stayed high and the lock was never cleaned up. Now it is decremented with try/finally.

• BaseException handling
  All exceptions now go through the same cleanup path (previously only Exceptions was caught).

• Frozen validation
  The check for the underlying cache no longer relies on assert (so it still works with -O).

• make_key
  Removed an unreachable branch.