(CR-4742) Move data from SearchParams to EntrySenders.

* because SearchParams aggregate data for both changeNumber based and cookie based searches (mix of concerns)
* it seems to me it would be more appropriate to use both EntrySender to store this information, for example it is storing the cookie too
* however I agree this blurs the role of EntrySender a bit, but not that much
* because storing these info in entrySender would more efficiently allow to avoid passing parameters in methods.




ChangelogBackend.java:
Moved SearchParams fields to the relavant entry senders: CookeEntrySender and ChangeNumberEntrySender. Only left highestChangeNumber and lowestChangeNumber for optimizations. Totally removed csn which was not used.
Renamed SearchParams to ChangeNumberRange.
Inlined initialSearch().
Inlined validatePersistentSearch() into initializePersistentSearch().
In initialSearchFromChangeNumber(), fixed a bug where we should stop after sending the base changelog entry if only based is asked.

ChangelogBackendTestCase.java:
Consequence of the changes to ChangelogBackend