This patch adds a beanstalk CEL backend.

Beanstalkd is a simple to use job queue. It provides a means to
create multiple job queues called "tubes". Each tube can store
multiple jobs, with varying priorities with the queue. Queue
processing is available via a simple TCP socket or via well defined
libraries, avaialble at
https://github.com/kr/beanstalkd/wiki/client-libraries

This module is based upon the beanstalk-client library, available
for download at: https://github.com/deepfryed/beanstalk-client

This module currently doesn't support user defined events.

Change-Id: Ic3a087faeeac045d69a2a018e60e29831ddb95ab