Select current store using a standard Config preference

[kennyadsl]

This is a first step in the attempt of reducing the queries made at each request in a default solidus store that doesn't use multiple stores, as for #1971 .

The goal here is to:

1. make it more standard/easier to use another class to determine which store to use (this PR)
2. move out of core the logic that checks requests info to determine if another store has to be used. This will be moved to solidus_multi_domain.
3. just use the Spree::Store.default on a solidus store without solidus_multi_domain avoiding multiple queries for each request.

[tvdeyen]

Really like that change

[mtomov]

That's just great! I always end up overriding Store.current to avoid the pesky lookups on every request:

  # class memoize it as we only have one store
  def self.current(domain = nil)
    @current_store ||= Spree::Store.default
  end

[mamhoff]

@kennyadsl would you mind rebasing this on the current master? The changelog changed again. :)

[kennyadsl]

@mamhoff sure, done!