Fix NULL token bug

Not sure how this escaped notice for so long, since it's been there from
the beginning.
WIP: slot abstraction, checkpoint
Add more type distinctions to Range

This allows a new type alias Section <: Segment representing a bounded
range of offsets.
Fix slotasset sql args after Volume condition changes

Introduced by 43b43b4
Merge branch 'master' into unslot

Conflicts:
	app/models/asset.scala
	app/models/slot_asset.scala
	app/store/asset.scala
WIP: slot abstraction, checkpoint
JsonRecord: remove quoting of string ids; include id in value
Include records in volume sessions, with age

Remove age from records.sessions.  Always use full record in value.
Merge branch 'master' into unslot
WIP: slot abstraction, SlotAsset now building
WIP: slot abstraction, some work on tags

A few more SQL utilities, still broken...