Host in trace info (fixes #847)

[jakajancar]

Currently, calling info.host() in trace and log filters does not work for http/2 connections, since they do not send the host in a header, but in the :authority pseudo-header. This change applies the same logic to info.host() as the one used for the host() filter. It also adds the host to the span of trace::request().

Note that there are two backwards-incompatible API changes:

The return type of filters::trace::Info::host(&self) changes from Option<&str> to Option<Authority>. The return type of filters::log::Info::host(&self) changes from Option<&str> to Option<Authority>.

[jakajancar]

@seanmonstar @jxs CR please

[jakajancar]

@seanmonstar This is still relevant to me, want to make some decisions on the open issues?