Implement release connection in batch

[EpicStep]

With this change, you can append to the batch directly without using helper structures (slice). Earlier this led to an empty pool, although it is not necessary to keep the connection in the batcher for the append operation.

In our implementation we saved about 2GB of memory per pod.

Close #1047

[jkaflik]

Thanks for your contribution. I understand the need for this; however not sure if we should expose ReleaseConnection, but do it for the user instead. This might be overhead for short-live batches.

Lets address comments and think if this approach is clean.

Fewer API functions we expose, the more clarity in the usage.

[EpicStep]

I agree with you. I think we can add options in PrepareBatch (it won't break backwards compatibility).
Example: PrepareBatch(context.Background(), "INSERT INTO some_table", WithReleaseConnection())

[jkaflik]

@EpicStep, your proposal sounds great

[jkaflik]

LGTM. Would you mind adding this feature to README?

[EpicStep]

@jkaflik Thanks. I added this feature to README.md

[EpicStep]

When will these changes be released?

[jkaflik]

@EpicStep I plan to release it today. In a merge train, your PR is last and has conflicts. It's easy, but it would be great if you push from your fork.

[EpicStep]

@jkaflik Yes, I can merge

[EpicStep]

@jkaflik done :)