fix: make HTTPDatastoreAPI compatible w/ microgen Gapic API (#136)

* tests: refactor to helpers for repeated tests * fix: update 'HTTPDatastoreAPI.allocate_ids' to follow request-only pattern Toward #124 * fix: add missing 'HTTPDatastoreAPI.reserve_ids' method Closes #134. * fix: update 'HTTPDatastoreAPI.rollback' to follow request-only pattern Toward #124 * fix: update 'HTTPDatastoreAPI.commit' to follow request-only pattern Toward #124 * fix: update 'HTTPDatastoreAPI.begin_transaction' to follow request-only pattern Toward #124 * fix: update 'HTTPDatastoreAPI.run_query' to follow request-only pattern Toward #124 * fix: update 'HTTPDatastoreAPI.lookup' to follow request-only pattern Toward #124 * fix: add 'retry' / 'timeout' args to HTTPDatastoreAPI methods Closes #124. * chore: lint * tests: run systests also w/ GRPC disabled. Closes #133.

Author: tseaver