Ticket 47577 - crash when removing entries from cache

Bug Description:  when the dn of an entry in the cache was adjusted to the parent dn, for soenm time
	teh dn was not defined, anothe thread accessing the dn of teh chached entry could crash

Fix Description:   hold the cache mutex when modifyingthe dn of an entry in the cache

https://fedorahosted.org/389/ticket/47577

Reviewed by: rmeggins
(cherry picked from commit 7272dbda7f43974eed003cbcfc0ddd57fe433687)
(cherry picked from commit 1056a6282f246a9c396b8052d726005fe8189512)
(cherry picked from commit 00b19f3b4af662341c72fa6cfd60b5e136f1428f)