OM-260: remove cursor consistency checks

Mutating cursors to bring them up-to-date wrecks havoc on the React/Om
model. We are not going to pursue this.

Instead we're simply going to push consistency issues onto the user as we
should have done in the first place. Now that we re-render when cursor
paths have changed consistency concerns should only arise in more
complex scenarios.

Users that need to check for cursor consistency can still use deref and
they have a new predicate om.core/valid? to detect whether a
cursor has been deleted.

Remove rendering? predicate - no longer serves a purpose.