Fix randomness in ChangelogBackendTestCase

ChangelogBackendTestCase.java:
In searchChangelog(), move the assert on the SearchOperation ResultCode inside the loop that asserts the number of returned search entries.

ExternalChangelogRequestControl.java:
In getCookie(), copied the cookie using new MultiDomainServerState constructor. This prevents unit tests from mutating the cookie coming from the control and making repeated calls with the same SearchRequest fail.

MultiDomainServerState.java, MultiDomainServerStateTest.java:
Added copy constructor + tests