MINOR: connection: add a new list member in the connection struct

MINOR: connection: add a new list member in the connection struct

This list member will be used to attach a connection to a list of
idle, reusable or queued connections. It's unused for now. Given
that it's not expected to be used more than a few times per session,
the member was put after the target, in the area starting at the
second cache line of the structure.