New things in the backend

[Ravi Pandya <ravi>]

I've been adding sensors and recorders and such to the backend, and I
have some questions about how they relate to the disk.

Sensors are session-level objects that have a single no-argument
no-wait message ("ring") that gets sent to them when the piece of
partiality they are attached to gets filled in. There needs to be some
way of removing them from the ent when they are deleted or when the
session ends. Right now they live in an ImmuSet inside a PartialLoaf.
Should they be dependents? What happens if the loaf gets sent to disk?
Should a loaf with sensors be prevented from going to disk?

HopRecorders are attached to a bert and fill in a partial orgl with
some information each time the bert is hopped. They should persist
across sessions. I don't know how one would get rid of a HopRecorder
once it`s created. A HopRecorder is a kind of Waldo, wrapping a Foot,
which grabs a Stamp on the partial orgl it is filling in. It is in an
ImmuSet of the Bert that it is watching. What does it take to make it
persistent?

BackfollowRecorders are similar, except that they are pointed to from
the sensor canopy.
	--ravi