OPENDJ-1116 Introduce abstraction for the changelog DB


Removed ChangeNumberIndexDB.getRecord(long).


ECLServerHandler.java:
Replaced calls to ChangeNumberIndexDB.getRecord() by calls to ChangeNumberIndexDB.getCursorFrom().
Inlined all the crossDomainStartState local variables.
Added getCursorFrom() to preserve old behaviour from JEChangeNumberIndexDBCursor ctor.


ChangeNumberIndexDB.java:
Removed getRecord().

JEChangeNumberIndexDB.java:
Removed getReadCursor(), getRecord() + some surprising and never used code like lock field, hasLock(), lock() and release().

JEChangeNumberIndexDBCursor.java:
In ctor, do not throw ChangelogException if the changeNumber cannot be found.

JEChangeNumberIndexDBTest.java:
Used JEChangeNumberIndexDB.getCursorFrom() instead of JEChangeNumberIndexDB.getReadCursor().
Renamed a few variables.