Namespace Configuration Env Vars with Backwards Compatibility

[tonyta]

Related to issue #1028.

Namespace all configuration variables used by Delayed::Job under the DELAYED_JOB_-prefix.

MAX_PRIORITY   👉   DELAYED_JOB_MAX_PRIORITY
MIN_PRIORITY   👉   DELAYED_JOB_MIN_PRIORITY
QUEUE          👉   DELAYED_JOB_QUEUE
QUEUES         👉   DELAYED_JOB_QUEUES
QUIET          👉   DELAYED_JOB_QUIET
READ_AHEAD     👉   DELAYED_JOB_READ_AHEAD
SLEEP_DELAY    👉   DELAYED_JOB_SLEEP_DELAY

This will allow developers to easily figure out what the env var is used for at a glance and prevent collisions with other libraries using similarly named variable names.

Allow non-namespaced configuration variables as a fallback, but with a deprecation warning.

Example usages:

With deprecated vars:

$ MAX_PRIORITY=1 \
* MIN_PRIORITY=1 \
* QUEUES=email,import \
* READ_AHEAD=5 \
* SLEEP_DELAY=25 \
* bundle exec rake jobs:work
[DEPRECATION] 'MIN_PRIORITY' for configuration is deprecated. Use 'DELAYED_JOB_MIN_PRIORITY'
[DEPRECATION] 'MAX_PRIORITY' for configuration is deprecated. Use 'DELAYED_JOB_MAX_PRIORITY'
[DEPRECATION] 'QUEUES' for configuration is deprecated. Use 'DELAYED_JOB_QUEUES'
[DEPRECATION] 'SLEEP_DELAY' for configuration is deprecated. Use 'DELAYED_JOB_SLEEP_DELAY'
[DEPRECATION] 'SLEEP_DELAY' for configuration is deprecated. Use 'DELAYED_JOB_SLEEP_DELAY'
[DEPRECATION] 'READ_AHEAD' for configuration is deprecated. Use 'DELAYED_JOB_READ_AHEAD'
[DEPRECATION] 'READ_AHEAD' for configuration is deprecated. Use 'DELAYED_JOB_READ_AHEAD'
[Worker(host:tonyta.local pid:42629)] Starting job worker
[Worker(host:tonyta.local pid:42629)] Exiting...

With namespace vars:

$ DELAYED_JOB_MAX_PRIORITY=1 \
* DELAYED_JOB_MIN_PRIORITY=1 \
* DELAYED_JOB_QUEUES=email,import \
* DELAYED_JOB_READ_AHEAD=5 \
* DELAYED_JOB_SLEEP_DELAY=25 \
* bundle exec rake jobs:work
[Worker(host:tonyta.local pid:42711)] Starting job worker
[Worker(host:tonyta.local pid:42711)] Exiting...