configurable client and server socket pools

[olix0r]

It's trivial to create a traffic loop:

while curl -H 'Host: woop' -H 'Dtab-local: /host/woop=>/$/inet/127.1/4140' localhost:4140 ; do :; done

💀 system runs out of file descriptors! 🔫

We need to consider other means to prevent/avoid loops, but this highlights the need to be able to constrain:

1. the number of sockets that a server may accept concurrently.
2. the number of sockets that a single client (not router) may open.

Finagle has stack parameters to control these. We need to ensure that there are sane default values, and expose a means to configure these parameters.