Ticket 48206 - Crash during retro changelog trimming
    
    Bug Description:  If the retro changelog entry is small, its possible that
                      during the trimming the reto changelog entry is not in the
                      cache after the trim, but its tries to blindly unlock it
                      from the cache, which leads to a crash.
    
    FIx Description:  After we call the post op plugins and retrieve the entry
                      from the cache, double check that it was found.  If it
                      is not found, do not unlock it.
    
    https://fedorahosted.org/389/ticket/48206
    
    Reviewed by: nhosoi(Thanks!)
    
    (cherry picked from commit 2a8a8c8ced5849dada34ab28d79e87dd3636e413)