let's use a fixed number of threads for libpager

[Justus Winter]

Hello :)

this is a new patch that makes libpager use a fixed number of threads,
currently one (plus one receiving messages and enqueuing them into a
userspace queue).  If our store interface improves, we can increase
the number of worker threads (the work delegation uses a linear scan
over the workers, so the number should be "smallish").

Preliminary testing on my box as well as on darnassus looks promising
(modulo some pfinet weirdness that Richard is investigating [and imho
if there is an issue, it should be unrelated to this change]).  We
will keep you posted.

Cheers,
Justus