Fix netdb exchange with a TLS cache_peer (#307)

Fix netdb exchange with a TLS cache_peer (#307)

Squid uses http-scheme URLs when sending netdb exchange (and possibly
other) requests to a cache_peer. If a DIRECT path is selected for that
cache_peer URL, then Squid sends a clear text HTTP request to that
cache_peer. If that cache_peer expects a TLS connection, it will reject
that request (with, e.g., error:transaction-end-before-headers),
resulting in an HTTP 503 or 504 netdb fetch error.

Workaround this by adding an internalRemoteUri() parameter to indicate
whether https or http URL scheme should be used. Netdb fetches from
CachePeer::secure peers now get an https scheme and, hence, a TLS
connection.