Singleton for blacklisting http client

[anoadragon453]

https://github.com/matrix-org/synapse/blob/bbf8886a05be6a929556d6f09a1b6ce053a3c403/synapse/handlers/identity.py#L55

Should be changed to hs.get_blacklisting_http_client, to allow for easier unit testing.

We should also stop saying it's the federation blacklist since we've applied it in many different places now.

... though there is an argument to being able to have multiple blacklists for different outgoing parts.