con_id member variable is shadowed in Http2ClientSession

Noticed this while tracking down another bug. The debug messages were including the con_id value which was always null even through the ProxyClientSession class atomic increments to create a new value on new_connection.

The member variable con_id is declared in both the parent class (ProxyClientSession) and the child class (Http2ClientSession). Aside from debugging the con_id member variable doesn't really seem to be used for Http2, so this isn't a critical bug.