Support PublishWithContext

[hedhyw]

Relates to #266 and #464.

The method adds the support of cancelling blocking operations in the Publish method. The PR replaces mutex with semaphore.

The changes are not breaking.

All integration tests are passed.

[michaelklishin]

Hey folks,

I'm posting this on behalf of the core team.

As you have noticed, this client hasn't seen a lot of activity recently.
Many users are unhappy about that and we fully recognize that it's a popular
library that should be maintained more actively. There are also many community
members who have contributed pull requests and haven't been merged for various reasons.

Because this client has a long tradition of "no breaking public API changes", certain
reasonable changes will likely never be accepted. This is frustrating to those who
have put in their time and effort into trying to improve this library.

We would like to thank @streadway
for developing this client and maintaining it for a decade — that's a remarkable contribution
to the RabbitMQ ecosystem. We this now is a good time to get more contributors
involved.

Team RabbitMQ has adopted a "hard fork" of this client
in order to give the community a place to evolve the API. Several RabbitMQ core team members
will participate but we think it very much should be a community-driven effort.

What do we mean by "hard fork" and what does it mean for you? The entire history of the project
is retained in the new repository but it is not a GitHub fork by design. The license remains the same
2-clause BSD. The contribution process won't change much (except that we hope to review and accept PRs
reasonably quickly).

What does change is that this new fork will accept reasonable breaking API changes according
to Semantic Versioning (or at least our understanding of it). At the moment the API is identical
to that of streadway/amqp but the package name is different. We will begin reviewing PRs
and merging them if they make sense in the upcoming weeks.

If your PR hasn't been accepted or reviewed, you are welcome to re-submit it for rabbitmq/amqp091-go.
RabbitMQ core team members will evaluate the PRs currently open for streadway/amqp as time allows,
and pull those that don't have any conflicts. We cannot promise that every PR would be accepted
but at least we are open to changing the API going forward.

Note that it is a high season for holidays in some parts of the world, so we may be slower
to respond in the next few weeks but otherwise, we are eager to review as many currently open PRs
as practically possible soon.

Thank you for using RabbitMQ and contributing to this client. On behalf of the RabbitMQ core team,
@ChunyiLyu and @michaelklishin.