chore: set batch size to 10 with graceful exit

This is related to the findings in gitlab-org/gitlab#508460 (comment 2284951017)

We noticed that having a batch_size of 10 in Snowplow tends to drop some events, so we reverted it back to 1.

This MR reintroduces a batch size of 10, with manual flushing if the application exits due to a new deployment.