[ADD] pos_cache: add pos_cache ported to v9 from odoo-extra

Originally written by Nicolas Seinlet <nse@odoo.com>. Provides
pos.config with a product.product cache.

This adds a ported and refactored pos_cache from odoo-extra:

- Adapt to changes in web and point_of_sale since 8.0
- Refactor the load_server_data function, so it isn't copy pasted but
  cleanly patched into the load_server_data super.
- Make the cron inactive by default.
- Also remove retrying the write 5 times, when it fails we'll just have
  to wait until the cron triggers again (an hour from then).
- Store the product.product fields and domain on pos_cache. When cache
  is requested from the pos with a different field or domain it'll
  automatically recompute. Necessary for when someone would install a
  module that modifies these things in the pos after a cache has been
  generated.
- Have multiple caches per pos_config per user
- Make the module adhere to the Odoo Coding Guidelines.