Change __wt_buf_grrow_worker to copy non-local data into place, this
simplifies some other code in the system.

Change buffer init calls to clear the buffer data size before calling
the grow function -- it shouldn't be necessary, but it clarifies what
is happening.
Kluge to make ex_call_center work in this branch, probably not the
correct fix; what's going on here is that if the key is a recno,
buf.data points at the cursor's recno field, and we're writing into
it, not into the WT_ITEM at all.
Revert 8795d9b, it makes things worse, not better.
Cleanup places where the schema layer uses a WT_ITEM to read/write from a record number.
Merge pull request #977 from wiredtiger/buf-grow-simplify

Some additional buffer management cleanup.