Avoid unpacking the key cell multiple times when returning key/value
pairs from row-store pages.

Enhance the cell unpack call to optionally return a reference to the
value cell that follows the key, so code that unpacks the on-page key
cell has the value cell without any further work.

Inline __wt_row_value (as __wt_row_leaf_value).
Merge branch 'develop' into cell-unpack-optimizations-2
Cherry pick 83b7068 from Michael's cell-unpack-optimizations branch.
Merge branch 'develop' into cell-unpack-optimizations-2
Avoid memset in performance-critical code by keeping a zeroed static structure instead.

refs #562
Merge pull request #837 from wiredtiger/cell-unpack-optimizations-2

Avoid unpacking the key cell multiple times when returning key/value pairs from row-store pages.