Revert "More key/value refactoring."

This reverts commit 2914c3f7146fb81c7023c91b8122e5857813325b.
Revert "Begin some refactoring of cursor key/value manipulation."

This reverts commit b3dcb902e8543be6639d0aec98f8fa94ccd3749a.
Use an underlying cursor for async op keys/values.
remove unused variables
Merge pull request #953 from wiredtiger/async.kv

Embed a cursor structure inside WT_ASYNC_OP so code can be shared.