Expose seastar::semaphore to Rust

[cvybhu]

seastar::semaphore is a seastar implementation of the semaphore primitive.

We would like to be able to use it from Rust.

One problem that might occur is that seastar::semaphore has methods which accept a timeout duration. The duration type is taken from a template Clock argument, which causes similar problems to the ones with seastar::timer (#9)
If it turns out that timeouts are a big problem we can start with implementing all the features that don't use timeouts and add the support for timeouts later.