Use unique names for queue unique cache configurations

[sheldonreiff]

Prevents an exception when multiple configs are set with uniqueCache configured.

Fixes #113

[markstory]

Couldn't this be put into the defaults so that userland code can define it if they really need to?

$cacheDefaults` = [
  'duration' => '+24 hours',
  'uniqueCacheKey' => "Cake/Queue.queueUnique.{$key}"
];

[sheldonreiff]

The whole array nested under uniqueCache is passed directly through to Cache::setConfig(), so maybe that would be slightly confusing that uniqueCacheKey isn't part of the cache config? We could bump it up a level though and let it be configurable as part of the main config. So configuring the unique cache would look like this:

'uniqueCacheKey' => 'someCustomKey', // (optional)
'uniqueCache' => [
    'engine' => 'File',
],

[markstory]

maybe that would be slightly confusing that uniqueCacheKey isn't part of the cache config?

Yeah that would be odd. Lets roll with what you have so far. If we come up with use-cases that require custom cache key configuration control we can revisit then.