Split HttpServer::AddRequestHandler into two methods

Spec requires handle only subset of  HTTPS paths on HTTP port.

BUG:24789091
Change-Id: Ic47818003f6a75ae3267a2d3a591d2576c73ac40
Reviewed-on: https://weave-review.googlesource.com/1277
Reviewed-by: Vitaly Buka <vitalybuka@google.com>