Use the correct bound for the semaphore in tower-batch

[teor2345]

Is your feature request related to a problem? Please describe.

tower-batch uses a semaphore bound of 1, but tower::Buffer uses the supplied bound:
https://github.com/ZcashFoundation/zebra/blob/main/tower-batch/src/service.rs#L60
https://docs.rs/tower/0.4.4/src/tower/buffer/service.rs.html#86

Describe the solution you'd like

We should use the supplied bound for the semaphore, so tower-batch can handle the expected number of concurrent callers.

Describe alternatives you've considered

Do nothing: the code might hang or otherwise be slow.

Additional context

This issue was discovered during the review in #1593. It could be a cause of the hangs in #1435, so it's a high priority.