R1:d16454d45d60

Improve a race condition in session establishment code

Summary:
If you try to establish several sessions quickly (e.g., by running several
copies of "arc" at once, as in "arc x | arc y"), the current logic has a high
chance of making them all pick the same conduit session to refresh (since it's
the oldest one when each process selects the current sessions). This means they
all issue updates against "conduit-3" (or whatever) and one ends up with a bogus
session.

Instead, do an update against the table…