Global workflow context

We need to implement a global context for Mistral workflows. Currently 'publish' keyword in Mistral saves variables into a storage which are associated only with a branch.

Example:

my_wf:
  tasks:
    A:
      action: std.noop
      publish:
        my_var: 1
      on-success: A1

    A1:
      action: my_action param1=<% $.my_var %>

    B:
      action: std.noop
      publish:
        my_var: 2
      on-success: B1

    B1:
      action: my_action param1=<% $.my_var %>

The expression "$.my_var" in the declaration of A1 will always evaluate to 1, for B1 will always evaluate to 2. This doesn't depend on order in which A and B will run. This is because we have two branches (A -> A1 and B -> B1) for which the variable "my_var" has its own different version.

Sometimes though we need to be able to pass data between branches. The concept of workflow global context can help solve this problem. A simple use case for this may be the following situation: one branch needs to notify the other one that it needs to stop.