Improvements to buffer pool

[lthibault]

• Removes unused buffer pool buckets
• Improves buffer reuse by consuming buffers even if capacity is not aligned to bucket boundary
• Aggressively reuses small-sized buffers by setting min page size to 1kb
• Improves buffer reuse by decreasing total number of buckets
• Small readability improvements (use range instead of classic for loop)
• Increase max page size to 1Mb

[zenhack]

A couple things in-line

[zenhack]

One more minor style nit

[lthibault]

I ended up re-writing a significant portion of this to allow users to provide their own pool implementations. I think this can have a very large impact in niche cases (e.g. embedded systems), where you might want to tune the pooling behavior to save memory. I can also imagine applications that allocate a large volume of very big segments.

I've also added tests around the sensitive parts of the default pool implementation.

[lthibault]

Weird, I'm getting build failures. Re-running 🤨

[lthibault]

Ok fixed. Ready for review.

[zenhack]

Some comments in-line

[lthibault]

Ok, I think this is finally ready to merge. How do you feel about squashing this branch?

[zenhack]

Once we figure out CI, SGTM.