Speed up light weight cursors.
Instead of asking the brt to use 'copyout' (by passing key and/or val pointers)
we pass in NULLs to brt_cursor_get.
We then later use the peek_current functions to obtain pointers to the current
key and val of the cursor.

That was worth ~22% reduction in time for scanscan --lwc --prelock --prelockflag

Also, now copyout quits immediately if both key and val are NULL
This increased the (total) value of the patch to ~30% reduction  in time.

git-svn-id: file:///svn/tokudb@5383 c7de825b-a66e-492c-adef-691d508d4ae1