Columnar faceting: HIERARCHY, DISTINCT, STATS, AFTER and facet.searchValues from Python #291

Part of #289 (columnar faceting in the Python bindings).

Scope

Make the later facet features reachable and tested from Python:

• HIERARCHY, DISTINCT (exact, and PRECISION n with exact: false), STATS (min, max, sum, avg) and AFTER paging of a terms facet ordered by value (MISSING on the last page).
• facet.searchValues('Type', 'property', 'prefix', limit) for typeahead over a string column.
• Non-finite and zero values through STATS and DISTINCT decode correctly (note: MIN/MAX of a negative zero come back as 0.0, a documented limitation).

Tests

One test per kind in the JVM and native lanes, including AFTER paging to the last page and facet.searchValues on a prefix with no match.