 - renamed class SegmentEntryForMerge -> SegmentIterator:  - made it lightwight by holding just shard_ptr to EntryIterator instead of having member, so there is not need to copy/move hevy EntryIterator object during heap balancing. (also this will from prevent heap defragmentation)
 - renamed class SegmentEntryForMerge -> SegmentIterator:
 - made it lightwight by holding just shard_ptr to EntryIterator instead of having member, so there is not need to copy/move hevy EntryIterator object during heap balancing. (also this will from prevent heap defragmentation)

 benchmarks shown about overall about 8 times merging speed improvement

cliqz/keyvi,hendrik-cliqz/keyvi,hendrik-cliqz/keyvi,hendrikmuhs/keyvi,cliqz/keyvi,narekgharibyan/keyvi,narekgharibyan/keyvi,cliqz-oss/keyvi,hendrikmuhs/keyvi,narekgharibyan/keyvi,narekgharibyan/keyvi,cliqz/keyvi,hendrik-cliqz/keyvi,cliqz-oss/keyvi,cliqz-oss/keyvi,hendrik-cliqz/keyvi,hendrikmuhs/keyvi,hendrikmuhs/keyvi,cliqz/keyvi,cliqz-oss/keyvi