Fixed a race condition in the registry. Introduced a new API call to unget ServiceReferences.

git-svn-id: https://svn.apache.org/repos/asf/celix/trunk@1620872 13f79535-47bb-0310-9956-ffa450edef68
Fixed incorrect(missing) pointer dereference

git-svn-id: https://svn.apache.org/repos/asf/celix/trunk@1620877 13f79535-47bb-0310-9956-ffa450edef68