substitute: Reuse connections for '--query'.

This significantly speeds up things like substituting the closure of a
.drv.  This is a followup to 5ff521452b9ec2aae9ed8e4bb7bdc250a581f203.

* guix/scripts/substitute.scm (http-multiple-get): Add #:open-connection
and #:keep-alive? and honor them.
(open-connection-for-uri/maybe): Use 'open-connection-for-uri/cached'
instead of 'guix:open-connection-for-uri'.  Call 'http-multiple-get'
within 'call-with-cached-connection'.
(open-connection-for-uri/cached): Add #:timeout and #:verify-certificate?
and honor them.
(call-with-cached-connection): Add 'open-connection'  parameter and
honor it.

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