Pushers tend to make many connections to the same HTTP host
(e.g. a new event comes in, causes events to be pushed, and then
the homeserver connects to the same host many times). Due to this
the per-host HTTP connection pool size was increased, but this does
not make sense for other SimpleHttpClients.
Add a parameter for the connection pool and override it for pushers
(making a separate SimpleHttpClient for pushers with the increased
configuration).
This returns the HTTP connection pool settings to the default Twisted
ones for non-pusher HTTP clients.
