Make !lock.readers and !lock.writers assertions safe.

Make !lock.readers and !lock.writers assertions safe.

The lock class used readers level counter to count both attempts to read and
current readers. The attempts part made assertions declaring that there should
be no readers unsafe because even a writing entry may have a reading attempt.
Same for writers counter: A reading entry may have a writing attempt.

We now segragate the attempts level, which is internal information required
for shared lock to work, from counting the number of successful attempts
(i.e., actual readers and writers), which is public information useful for
assertions, stats, etc.