Fixed an open-database-count error in CBLManager
Fixed an open-database-count error in CBLManager

It was possible for a database instance to be unregistered twice from
CBL_Shared, making the -countForOpenedDatabase: wrong. I saw this happen
in unit tests when calling first -close and then -deleteDatabase: on
a CBLDatabase.

couchbase/couchbase-lite-ios,amco/couchbase-lite-ios,amco/couchbase-lite-ios,couchbase/couchbase-lite-ios,amco/couchbase-lite-ios,amco/couchbase-lite-ios,amco/couchbase-lite-ios,couchbase/couchbase-lite-ios,amco/couchbase-lite-ios,amco/couchbase-lite-ios,couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios