subqueries should be advised, explained and monitored

A placeholder to track an annoyance in N1QL: subqueries are planned as part of execution, and thus are not explained.
Also, since the request plan is missing the subquery, they are not monitored.
We should have a way to hang a subquery plan to an expression.
And ideally, we should have a cache of plans so that we don't reprepare over and over again.