lock-free radix set for storing page IDs

[spacejam]

we're super space inefficient when storing sets of page IDs in the SegmentAccountant, currently using BTreeSets and HashSets.

the bitset implementation needs the following properties:

• starts in "grow-only" mode
• after stabilizing the segment, switches to "shrink-only" mode
• will be a u64 with at most 42 bits used
• lock-free adds when in grow-only mode
• adds may be eventually consistent
• lock-free removals when in shrink-only mode
• removals may be eventually consistent

I'm tempted to use a radix trie-based approach instead of something like roaring bitmaps because I don't know how to approach making bitmaps like that lock-free. The eventual consistency tolerance means we can cut a lot of corners where normal lock-free data structures would need to be more responsible for achieving linearizability.

inspiration:

• WORT: write-optimal radix trie
• HOT: height-optimized trie