SLING-3647 - No repository found at ... URL scheme http not supported.
only

Since the RepositoryProvider is not thread-safe, explicitly synchronize
access to it. This potentially fixes the bug, but it remains to be
validated.

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1600759 13f79535-47bb-0310-9956-ffa450edef68