General refactor of the package

[prep]

This PR is a general refactor of the package. It includes the following things:

• The consumer now only reserves a job when there is a worker ready to accept it. This means that the package no longer offers an exported channel on which you can receive jobs. Instead, everything is handled via the Receive() method.
• A cancel of the context passed to Receive() will not cancel the in-transit jobs. Instead, those are allowed to finish before Receive() returns.
• Log messages are now being done by calling a configurable function. This allows the implementer to hook in their preferred logging solution.
• ProducerPool and ConsumerPool (and their New* functions) are removed in favour of just a Producer and Consumer.
• The NewProducer and NewConsumer functions will no longer return an error if they were unable to set up a TCP connection to the beanstalk server. This caused problems in scenarios where not all beanstalk servers are available all the time.