Change callers of __wt_row_leaf_key_work to always pass in a buffer, it's
an easy change for a minor simplification.
Revert fb25f0f, instantiate keys accessed during random search, on a big
page where the page's keys have a common prefix, it's a really bad idea
to repeatedly build the keys.

Remove the looping code in __wt_row_leaf_key(), it's no longer necessary.
Move the language private reference to the top of the structure, I end
up chasing a core dump every time the WT_SESSION_IMPL structure changes
and I forget to install the library before running the test suite, ref