OPENDJ-1623 NullPointerException while doing psearch (with changesonly=false) on cn=changelog

Problem introduced by r11087. 

ChangelogBackend.java:
In initializePersistentSearch(), always create the ChangeNumberRange object.
Problem is that with changesOnly=false, this object will be created twice: one from ChangelogBackend.registerPersistentSearch() and another from ChangelogBackend.search().
I really think we should have a unique Backend.preSearch() method which is always called regardless of whether this is a persistent search or not.
This would allow to factorize search initialization in a single place, before it can be used to return entries by the initial and persistent phases.