Added ziplistDeleteRangeFromOffset allowing to remove more than one entry from ziplist starting from a given entry in the ziplist.
Changed ziplistDelete to return result from ziplistDeleteRangeFromOffset call.
In addition, replaced every ziplistDelete double-call to a single ziplistDeleteRangeFromOffset call.
These changes were made due to a "TODO" request in the code.
Tests - I have run make test to approve that my changes haven't caused new problems.