Fix #418 deleteDatabase: blocking thread

* CBL_Shared's forgetDatabaseNamed: blocks CBLDatabase objects from closing when they receive DatabaseWillBeDeleted notification. Instead of doing sleep, calling runloop to spin to allow the other CBLDatabase objects to get closed.

* When closing CBLManager, closing all databases in the CBL_Shared object as well.
#418 Revert changes in CBL_Shared and update unit test

* Per code review, revert changes in CBL_Shared back to use usleep instead of using NSRunLoop to wait.

* Update API_DeleteDatabase to ensure that the copied manager is closed and add test for the case that multiple managers working on different threads.
Merge pull request #428 from couchbase/feature/issue_418

Fix #418 deleteDatabase: blocking thread