Just an attempt at sending actual chunks of key/values rather than just one key/value pair when backfilling.
Still lots of TODOs and hacks.
Pass in scoped_ptr_t<superblock_t> instead of superblock_t*. Use batched replace when receiving a backfilling chunk.
Addressed a few TODOs in the backfill chunk code. But still needs rewrite in one part.
Rewrote backfill chunk processing logic.
Minor cleanup.
Fixed a wrong ALLOCATION_CHUNK parameter.
Parallelize backfill chunk inserts (partially).
Use spawn_now_dangerously for better efficiency.
Revert "Use spawn_now_dangerously for better efficiency."

This reverts commit 08e715e5a503116bca24aa6dde8252855a689579.
Revert "Parallelize backfill chunk inserts (partially)."

This reverts commit 5e1e3231ed9c19b061a4c20f8528ade05354ccc1.
Introduced a size limit for backfilling chunks.
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.
Wrong BACKFILL_MAX_KVPAIRS_SIZE
Bug in backfill chunking code
And the reamining part of the bugfix.
Fix JS driver and test ql-js-connect
style
Fix build on lucid

Review 1232 by @srh
Closes #1967