Dataframe v2: big ol' hindsight-driven refactoring (#7683)

Simpler. Faster. Correct-er.

* Fixes #7681
* DNM: requires #7677

Author: teh-cmc

Cargo.lock

@@ -5134,7 +5134,6 @@ dependencies = [
  "anyhow",
  "itertools 0.13.0",
  "nohash-hasher",
- "parking_lot",
  "re_arrow2",
  "re_chunk",
  "re_chunk_store",

crates/store/re_dataframe/Cargo.toml

@@ -38,7 +38,6 @@ anyhow.workspace = true
 arrow2.workspace = true
 itertools.workspace = true
 nohash-hasher.workspace = true
-parking_lot.workspace = true
 
 [dev-dependencies]
 re_types.workspace = true