Don't count unused arena bytes as read_bytes in AggregateFunction types

[Algunenano]

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

• In AggregateFunction types, don't count unused arena bytes as read_bytes.

Documentation entry for user-facing changes

Several changes:

• Reduce a bit the dependencies over the Arena.h header (to make it a bit faster to iterate, but not much).
• Add a new member in Arena to differentiate between allocated bytes (pre-allocated in the Arena or wasted in holes) and reserved_bytes (bytes requested via allocations to the arena).
• Change ColumnAggregateFunction to use reserved_bytes() when reporting the column ::byteSize(), leaving allocatedBytes() the same. This allows more precise reporting of read_bytes when working with these column types.

Closes #48606

[Algunenano]

All green. I didn't know this was possible

[Algunenano]

It'd be great if somebody could review this

• Failures: Integration tests (tsan) [3/6] hdfs failed to start

[alexey-milovidov]

"Reserved" bytes sounds misleading.

It is expected that it means something similar to std::vector::reserve and std::vector::capacity, but the meaning is the opposite.

What do you think about renaming it to "used" bytes?

[Algunenano]

What do you think about renaming it to "used" bytes?

Makes sense to me. I've changed it in the PR