Replaced PROTO_SSL_PEEK with request_flags::sslPeek and disabled server SNI

Replaced PROTO_SSL_PEEK with request_flags::sslPeek and disabled server SNI
for bump-server-first connections.

While PROTO_SSL_PEEK was a safer design option because requests with the
"wrong" protocol scheme would be less likely to leave Squid, it required
all error-generation code to replace the protocol with PROTO_HTTPS so
that error make more sense to end users. We no longer have to do that.

The server-side SNI for bump-server-first connections has to be disabled
because bump-server-first code does not yet know the true intended server name
(even for those CONNECT requests that have server name, it would be a little
risky to use CONNECT info for SNI). That name could be eventually obtained
from the client before we peek at the server certificate but that work
is outside this project scope.