Deprecate request::content

[xemul]

trafficstars

It's an sstring field that is tempted to be converted into temporary_buffer. However, it's the part of the API and other may still use it (alternator doesn't), so it has to be deprecated explicitly.

refs: #1345

[avikivity]

Looks good.

[xemul]

upd:

• made httpd::connection be a friend of request so that the newly introduced read_body() can be made private