Add a query planner for OxQL

[bnaecker]

• Add types to represent the plan tree and the currently-supported plan tree nodes. These mostly correspond to the existing query AST nodes, but include information about the expected schema for the input and output tables, along with the query AST nodes that "implement" that transformation.
• Add an explicit node for computing deltas from a cumulative timeseries, automatically after the node for fetching its data from the DB. This is currently implicitly done after fetching the data, but will be part of an explicit plan step going forward. The ultimate goal is to push that into the database itself where possible.
• Adds methods to optimize a query plan, which currently includes the predicate-pushdown and limit-pushdown tricks we already do to limit the amount of data we get from the database. Adds some tests to verify behavior of these optimizations, in particular that they don't change the planned output of the query itself.
• Add pretty-printing of the plan tree, and include a way to show that in the OxQL shell.
• Add detection of full table scans. Use the planner in OxQL queries, only to verify them and check that there are no scans. The queries themselves are executed in the original method today.