add queue subscriber and unit test

[stevelr]

add queue subscription and unit test
there are a couple open questions on function naming (see TODO in code)

Signed-off-by: Steve [email protected]

[stevelr]

I didn't want to stray from your existing conventions, but it would be slightly more ergonomic for the signature of subscribe to be
fn subscribe<T:ToString>(&self, subject: T)
and for queue_subscribe to be
fn queue_subscribe<T1:ToString,T2:ToString>(&self, subject: T1, queue_group: T2)

Or, if they are to be unified into a single subscribe function, then
fn subscribe<T1:ToString,T2:ToString>(&self, subject: T1, queue_group: Option<T2>)

If you're ok with either of those, I can also implement it on the next commit.

[Jarema]

ToString allocates, even if you pass String.

Into<String> is a better option here, though we decided to NOT have that right now, as we might add Subject type later, which would break API.

We refrained from having those helpers to allow ourselves to decide later.
Both @caspervonb and I want to be very clear in the codebase - especially in publish which can happen thousands of times per second - what is allocated etc.

[caspervonb]

I'm hesitant towards doing this because the ToString implementation for String is a deep clone, it could be Into<String> which should be a move but personally I'm more for explicit conversions than implicit ones as the caller is in control of how the conversion happens.

We're considering if we want a (potentially interned) type for Subjects, header names will likely get one in the form of HeaderNames from the http crate's API.

I'd say hold off on this for now, at-least until the subject-type matter is resolved.

[MattesWhite]

Short notice: So far #222 is ready for review and completely integrated the Subject type and a AsSubject trait into the old code base. It may went unnoticed with all the async rewriting process.

[caspervonb]

Hey @stevelr, appreciate the test driving, feedback and PRs! 😁

LGTM