Dependees of Uncacheable nodes should be cleaned once per Session

[stuhood]

Currently, the UncacheableDependencies state is the same as the Dirty state, with two differences:

1. In poll calls, we don't re-run UncacheableDependencies nodes unless they are marked Dirty.
2. Dirty nodes go to the Clean state if their dependencies have not changed, but an UncacheableDependencies node goes back to UncacheableDependencies after being successfully cleaned.

The latter point is a potential silent performance issue. Because an UncacheableDependencies node does not go to the Clean state for the current session, any non-concurrent request within the same session for an UncacheableDependencies node will cause it to be re-cleaned (recursively down to the Uncacheable node that is somewhere below it). While cleaning nodes is fairly cheap (and the Uncacheable node will correctly one run once per session), it's possible that we will repeatedly re-clean an UncacheableDependencies node within one session... in the worst case this could involve n! node-cleaning-visits for a path of length n between a root and an Uncacheable node, although that should be almost impossible in practice due to the likely shape of the graph.

To fix this, we'll likely want to incorporate the SessionId into the UncacheableDependencies state (or allow a node to be marked "Clean but only for a particular session"), similar to what we do for Uncacheable. This would allow the UncacheableDependencies node to run once per session like Uncacheable does.

(as a medium term tangent: it's likely that the SessionId that we have on nodes should act more like a MVCC timestamp, which would allow for concurrent runs of a node for different sessions to have different values.)