Implement an allocator that can reuse allocated chunks.

[ziyan]

Improve the memory pool allocator to reuse its allocated chunks.

1. keep chunks in a doubly linked list
2. on Clear(), set size to 0 for all chunks
3. when allocating space, reuse previously allocated chunk
4. when allocating larger space than previous chunks, find or allocate next available chunk and reorder the doubly linked list to reduce waste (I assume this rarely rarely happens, default chunk size is 64kB)

Assumption: Malloc() requested size is smaller than default chunk size in majority of cases.

Small program to demonstrate

#include <fstream>
#include <rapidjson/document.h>
#include <rapidjson/istreamwrapper.h>

int main() {
    std::ifstream ifs("bigfile.json");
    rapidjson::Document doc;
    for (size_t i = 0; i < 1000; i++) {
        // seek back to the beginning of the stream
        ifs.clear();
        ifs.seekg(0, std::ios::beg);
        rapidjson::IStreamWrapper isw(ifs);

        // reset the doc, or use a secondary doc and swap like flip-flop when parsing
        doc.SetNull();
        doc.GetAllocator().Clear();

        // parse
        doc.ParseStream<rapidjson::kParseFullPrecisionFlag>(isw);
    }
}

g++ -o testrapidjsonparse testrapidjsonparse.cpp

Size of the bigfile.json is 1.9MB. Loading it 1000 times only resulted in 33 allocation calls in the first parse.

heaptrack stats:
	allocations:          	33
	leaked allocations:   	0
	temporary allocations:	1

Before this PR

heaptrack stats:
	allocations:          	37008
	leaked allocations:   	0
	temporary allocations:	15000