Revert "Revert "Parallelize backfill chunk inserts (partially).""

This reverts commit 2b8cc74baff48b1865116a3ed0f76ed5327fe774.

Conflicts:

	src/clustering/immediate_consistency/branch/backfiller.cc
Revert "Revert "Use spawn_now_dangerously for better efficiency.""

This reverts commit cc15310b3cf45bc47462ce8e0ba95b7096d02ecf.
Removed a TODO that's not really that important.