Coalesce empty objects in HdrHeap
Coalesce empty objects in HdrHeap

HdrHeap could be filled by `HDR_HEAP_OBJ_EMPTY` type objects in some cases especially HPACK.
In this situation, `HdrHeap::required_space_for_evacuation()` & `HdrHeap::evacuate_from_str_heaps()`
could be slow because these functions always walk through `HdrHeapObjImpl` one by one.
Coalescing empty objects next to each other can reduce number of objects and mitigate this issue.

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