check performance of allocation of big blocks by mem_root under windows

Under windows allocation more than 16KB could be expensive.

Benchmark stop growing memory blocks more than 16KB in MEM_ROOT.

There is an alternative system allocator on Windows, called Segment heap, which in my tests always beat the default "low fragmentation" heap, which also does not have problems with larger 16K blocks. It is relatively easy to enable via application manifest (which we already use for some purposes, e.g enabling utf8 codepage by default).

This should be investigated, and benchmarked, to compare to alternative( to reducing some default MEM_ROOT sizes)