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.
You need to call [copiedMgr close] when you're done with it, otherwise it stays open for the whole run of the unit tests, which can cause problems.