Reduce the max pending forwarded gossip messages somewhat

Reduce the max pending forwarded gossip messages somewhat

There's no reason to forward more gossip messages than we include
in the pending buffer during initial sync. Forwarding is a
best-effort action and we don't care too much if we have to drop
messages because we're receiving too many gossip messages, in fact
that's the desired otucome.

Initial gossip, on the other hand, we want to be as liberal as we
can be, as we are effectively rate-limited to our messages-per-ping
messages per our network RTT, which can lead to initial sync being
rather slow.