this map is useful to track what namepace path different listeners
are using when, for example, using the same internal path (i.e. peeking
for the same path using the %publ namespace with -meen, but also
with the per-peer %chum namespace used in -chum).

Here we track also what types of interest gifts different listeners expect.
Originally this was either %tune (for |fine) or %sage (for |mesa) but we also
add a %rate gift so subscribers can be notified of the progress of the +peek.

(WIP: %yawn/%wham paths to cancel peeks)

all are subsumed by -peek

tl;dr: don't migrate the next seq number into the flow if this is
a naxplanation bone, and make sure that we don't replace the
contents of the flow if we later encounter a naxp bone, after
having processed the reference one

(a backwas flow gets automatically deleted if it's actually peeking for a $cork)