add more unittests for compatible-first locking policy

When MongoDB switches to compatible-first locking policy, it needs to apply this policy to new incoming requests as well as currently pending ones. We should have unit tests that test the following scenarios:

• Policy changes as result of pending MODE_S request being granted
• Policy changes as result of granted MODE_X request being downgraded
• Unblock pending MODE_IS request queued behind MODE_IX
• Unblock pending MODE_IS request queued behind MODE_X