The backend fold_objects was failing because it was trying to use
wterl:fold_keys/3, which doesn't fold over values. Add fold/3 to
wterl.erl to fix this.
Also clean up a few dialyzer warnings.
WiredTiger session count defaults to 50, but we need at least one per
vnode, so for now hard-code it to 100. Also ignore table creation
results in the kv backend because WiredTiger is currently producing a
strange error if the table already exists.
Add app, supervisor, and worker to open and cache the database
connection. Revert to using binaries for WT config in order to allow
wterl callers to use proplists for configuration. Change {error,
not_found} to not_found return values from wterl.
