limiter: refactor to reduce code duplication

limiter: refactor to reduce code duplication

PlackLimiter, Qspawn, and ViewVCS all have roughly the same
code around our base Limiter package, so put everything around
a new Limiter->may_start subroutine.  PlackLimiter loses some
stats as a result but that's logged anyways and I doubt the
customizable error message was worth the effort.

We now have ckhup and 499 (client disconnect) handling for all
PSGI uses of Limiter, as well.

t/qspawn.t changes were required since the original ->finalize
logic now relies on on_destroy; but none of the existing PSGI
code using Qspawn required changes.