The official `latest` Docker image for InfluxDB is now a 2.x image. That image now supports initial provisioning via environment variables like the 1.x image does.

This makes the following changes:
- Explicitly use a v1.x image (latest is 1.8) for the v1 tests
- Use the DockerHub image for v2 tests
- Use the new image's built-in provisioning support to simplify the Makefile
- Use a hard-coded token to avoid having to inject it everywhere dynamically in the tests
- Revert back to using `/api/v2/delete` for cleaning up after tests. It is now supported again, and the v1 compatibility endpoint wasn't working the same way with the latest version.
- Remove the `.token` file from `.gitignore`; we no longer generate this file.

When sending events via HTTP, the `worker_pool` library is used with a hard-coded number of workers (3).

This allows the number of workers to be configured by the client application. The default remains 3 if the client doesn't specify a value.