src/client: Make subscribe stream capacity explicit

[mxinden]

Makes footgun on Client::subscribe explicit.

Note that once the capacity of the returned [Stream] is reached, the
background task blocks and thus all work related to the [Client] will
pause until elements from the [Stream] are consumed and thus capacity
is freed. Callers of [Client::subscribe] should either set a high
capacity, continuously read from the returned [Stream] or drop it.

This has probably cost me 1h to debug on libp2p/test-plans#26. What do folks think?

[ackintosh]

Looks good to me. 👍

[laurentsenta]

Nice, is this the reason why tests are hanging on failures?

The Testground instance failed with:

Aug 24 04:57:08.754201	INFO	0.6544s      ERROR << single[000] (cbda45) >> thread 'main' panicked at 'a semaphore may not have more than MAX_PERMITS permits (2305843009213693951)', /usr/local/cargo/registry/src/github.zerozr99.workers.dev-1ecc6299db9ec823/tokio-1.20.1/src/sync/batch_semaphore.rs:134:9

https://github.com/testground/sdk-rust/runs/7987701480?check_suite_focus=true#step:10:111

[mxinden]

Nice, is this the reason why tests are hanging on failures?

No. This should not be related to any CI failures.