wrapper functions

The inner loop of `couchstore_save_documents` in every wrapper -- except ForestDB -- has a malloc/free pair and a bunch of memory copies.

Every wrapper -- except ForestDB -- gets two more malloc/free pairs and associated memory copies in the inner loop of `couchstore_walk_id_tree`, and WiredTiger gets another malloc and memory copy in `couchstore_open_document` nobody else has.

I won't speak for LevelDB or RocksDB, but any data copy is almost certainly unnecessary in WiredTiger's case.

At the least it would be utterly trivial to limit the malloc/free calls to a single call per set of documents or cursor traversal, or why aren't these buffers allocated per cursor?