Deprecate paths in FROM clause

For various reasons, N1QL needs to deprecate support for paths in the FROM clause. (1) They produce results and cardinalities that are inconsistent with non-path FROM clauses, e.g. if the path produces some MISSING values. (2) The dot notation for paths should be used for namespaces and / or collections instead.

Deprecating paths in the FROM clause will require a few steps over the next few months. First, we should deprecate them in documentation. Second, we should remove them in collateral and training. Next, we should add a warning or error out in a pre-Spock minor release. Finally, we should make the final switch to collections in Spock.