FLANN allocates huge amounts of memory with every call to findNeighbors due to its private heap allocations. This affects memory throughput and overall speed of user applications, especially the ones that frequently query for neighbors. After all, although malloc does cache on its own, it also does a lot of smart things besides that, which costs some time penalty. To solve the problem, this PR introduces a static, thread-safe, and bounded memory pool for heap allocations. It have been tested thoroughly for very long hours, under extensive parallelization, and with strict leakage profiling. The tests showed more than 70x memory throughput gain. For roughly the same amount of calls, the memory footprint of findNeighbors using master branch was around 18.2 GB/sec, which got reduced by this PR to just 250.5 MB/sec (see the allocations throughput table below). Additionally, this PR cleans up some unnecessary variables in the cvflann::Heap class.

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Pull Request Readiness Checklist

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• N/A: There is reference to original bug report and related work

Already existing:

• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

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