Allow package to be built without zookeeper

[begriffs]

I am using sqlite but was forced to install zookeeper on my system in order to build the jobqueue package. Is there a way to encapsulate this dependency the way HDBC does with its various adapters?

[ikehara]

I also want that to support various backends but it needs package and API refactoring. The current API allows backend instantiation based on the scheme part of URI-like string but I'm afraid that it is difficult to realize this without depending on DB libraries. I think some API change will be needed. Anyway, I will check what other libraries do for this.

[cdepillabout]

I am +1 on this. I am in the same boat as @begriffs. I was forced to install zookeeper even though I'm only using sqlite.

[junjihashimoto]

I am +1, too.
We do not use sqlite for our product.
Currently, update-plan is nothing.
Pull request is very wellcome.