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.