MINOR: connection: support PROXY v2 TLV emission without stream

MINOR: connection: support PROXY v2 TLV emission without stream

Update API for PROXY protocol header encoding. Previously, it requires
stream parameter to be set. Change make_proxy_line() and associated
functions to add an extra session parameter. This is useful in context
where no stream is instantiated. For example, this is the case for rhttp
preconnect.

This change allows to extend PROXY v2 TLV encoding. Replace
build_logline() which requires a stream instance and call directly
sess_build_logline().

Note that stream parameter is kept as it is necessary for unique ID
encoding.

This change has no functional change for standard connections. However,
it is necessary to support TLV encoding on rhttp preconnect.