AMQP plugin: prefer the `rabbitmq-c/amqp.h` header if available.

AMQP plugin: prefer the `rabbitmq-c/amqp.h` header if available.

At some point, RabbitMQ has moved their headers to the `rabbitmq-c/`
subdirectory. The old locations still exist but throw an error, saying
the old headers are "deprecated".

This adds appropriate checks for the new headers to the configure script
and uses those if present.

To simplify both the configure script and plugin, support for ancient
versions of the library is removed. This affects versions that don't
have the `amqp_tcp_socket_new()` function yet.