Realm: Improve fragmentation caused by basic range allocator

[apryakhin]

That shouldn't be a duplicate issue as far as I can tell. @lightsighter proposed and implemented a replacement for realm's basic range allocator:

• https://gitlab.com/StanfordLegion/legion/-/merge_requests/1394

The basic range allocator maintains a single unsorted list of free blocks. The incoming allocation always grabs the first entry from the free list that is big enough for the requested size and returns the remaining part as a new free block. Although this strategy is simple, it can result in memory fragmentation.

As part of this GitHub issue, we need to run some tests, add benchmarks on the "Size-Based Range Allocator" possibly work on replacing the existing one. Ideally, we should find a real-world existing application that this would help improve.