Dataframe v2: "constant-time" pagination (#7665)

This implements support for "constant-time" pagination.
Obviously it's not constant-time, but it scales in a sane fashion.

This PR is still _not_ about general performance optimizations. It is
the last step before those can start though.

* Fixes #7657 
* DNM: requires #7652

Cargo.lock

@@ -5131,7 +5131,6 @@ dependencies = [
 name = "re_dataframe"
 version = "0.19.0-alpha.1+dev"
 dependencies = [
- "ahash",
  "anyhow",
  "itertools 0.13.0",
  "nohash-hasher",

crates/store/re_dataframe/Cargo.toml

@@ -34,7 +34,6 @@ re_tracing.workspace = true
 re_types_core.workspace = true
 
 # External dependencies:
-ahash.workspace = true
 anyhow.workspace = true
 arrow2.workspace = true
 itertools.workspace = true