Optimize mask merging and improve memory use

[zhiltsov-max]

Summary

• Added more memory control for mask merging and mask materialization operations
• merge_masks can now receive an iterator, which can be helpful for reducing memory consumption
• Improved the memory situation in and fixed Dumping Segmentation Masks 1.1. takes way too long #53
  • Memory for masks should not be an issue anymore
  • There is still can be a problem with low performance, when merging lots of masks (or polygons into masks)
• Significantly improved performance:
  • of masks_to_polygons (10k x 10k image & 250 masks experiment: 200s -> 30s)
  • of CompiledMask building (10k x 10k image & 250 masks experiment: 200s -> 80s)

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT