WL#4535: QFPD: Storage Engine Pushdown API (SEPD API)

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WORK IN PROGRESS
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This task should implement the Storage Engine API needed to
support Query Fragment Pushdown. The current agreement at the
architecture meetings in 2008 is that we need to implement
the following functionality:

* handler::qf_cost() - interface to costing/optimization facility of an SE
* handler::qf_prepare() - interface to query plan generation facility of an SE
* representation and negotiation of capabilities
* more?