Undo deprecation of connection value
In #798 the connection value has been deprecated with the claim it is
useless. However, in fact it is used at least with the Mirage backend in
order to track the connections for logging etc. (See last two comments of
the PR.) In over 3 years no replacement has been added, so this change
removes the deprecation until some alternative exists. (Which could be
possibly Mirage only.)