Make FederationReaderServer _http_listen use self.get_reactor()

For all the homeserver classes, only the FrontendProxyServer passes
its reactor when doing the http listen. Looking at previous PR's looks
like this was introduced to make it possible to write a test, otherwise
when you try to run a test with the test homeserver it tries to
do a real bind to a port. Passing the reactor that the homeserver
is instantiated with should probably be the right thing to do anyway?

Signed-off-by: Jason Robinson <jasonr@matrix.org>