Callback map sharing

[endremborza]

Hello! this is the idea referenced in #787 I thought I might as well show you what I'm on about. This gives me the chance to solve issue 2 there.

There is a lot here, but in the end, there is no significant change to currently running and tested code. On the other hand the new functionality can only be tested locally yet. ( circleci/python:#.#-stretch-node-browsers does not have a redis server set up, so i expect the ci tests to break. sorry for that, if this direction is approved, that obviously needs to be fixed)

I am merely suggesting, that this is a possible way towards the the feature of being able to dynamically add interactive elements in dash, which I find to be quite powerful. I'm working on a few projects involving dash, and all of them would greatly benefit from something like this, so if you have suggestions or some ideas how this can evolve into something that can be merged, I'm willing to put in the work.

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • validate_callback_output function needed to be moved from @staticmethod to top level function to allow dill pickling. it is not used anywhere else
  • callback_map assignment needed to be changed, this way it's elements can be immutable
  • a RedisDict class added and unit tested to optionally replace the dict type of callback_map
  • tests for the redis callback_map and multiple output callbacks (this was also a necessity, as multiprocessing.Value does not work with dill pickled functions)
• I have run the tests locally and they passed. (refer to testing section in contributing) - since the linked file does not work as dev-requirements.txt does not exist, I needed to improvise
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR
  • unit test for the redis dict
  • simple callback tests for redis callback map

[alexcjohnson]

Thanks for advancing the discussion with this POC @endremborza - I'm going to close this as it's become stale, but feel free to follow up in #787 if there are further use cases not addressed or awkward using #1103