Cache declared entities using a unique key instead of hash.

[rectalogic]

Also build the key for Queues using bindings so that bindings are properly declared.

This is a fix for celery/kombu#396

[coveralls]

Coverage increased (+0.03%) when pulling d4df9aaa9ec554dcf726990b52328d1e01819b3f on rectalogic:declaration_key into 82dc37b4c151a2d4e7eec1f96ada5babc8749e8e on celery:master.

[georgepsarakis]

@thedrow @auvipy should we plan on an implementation based on the work of this PR? Seems quite important.

[auvipy]

yes why not :)