v0.5.1 -- Remove the unused Elasticsearch store

Removes the unused Elasticsearch store. No other change to behavior.

What changed since v0.5.0:

• Removed palimpsest/store.py (ElasticStore) and the elastic extra. It was built early and kept in case scale ever demanded it. It never ran against a live cluster, could not back consult() or Memory (it lacks all_nodes() and all_edges()), and nothing in the project used it. SQLite is the store everything runs on and handles a curated memory's size without a server. The decision and its reasons are in DESIGN.md ("Settled: no server-backed store").
• Breaking only for anyone who imported palimpsest.store directly, or installed palimpsest[elastic]. v0.5.0 and earlier still contain it.
• Benchmark housekeeping: a technique field on the eight forgery cases (description for reporting, not an input to any judge), and the design doc's answered questions and case counts brought up to date.

700 tests pass, 3 skipped.