Filter inbound connections

[Stebalien]

Transports should apply the "address filters", either directly on all incoming packets or, at least, on the initial handshake packets. Applying them on all incoming packets could be a performance hit at the moment as the filter is very unoptimized.

The transport can get a copy of the filter set by accepting a *github.com/libp2p/go-maddr-filter.Filters in the constructor.

[marten-seemann]

The easiest solution will probably be to apply the filter to all Long Header packets.