Extract "global" getters and setters, logging code, etc.

As a first step toward enabling multiple-site deployment, we've externalized the code for getting and setting "global" variables such as current pid, current target server, sessid, etc.

These getters and setters have been added to the namespace "deploy_push_*", since they relate to a specific "push" of a given deploy plan.

We've also started refactoring to use session variables instead of Drupal variable_set/get variables in order to reduce the DB load. Current code includes some parallel use of $_SESSION, leaving Drupal variable use intact so that all legacy code will still be compatible. Ideally, other modules could use take of interface commands like deploy_push_get_current_pid() without worrying about the underlying implementation.

I'll post the patch as a comment, once this issue has a #. We'll post additional multi-site support building on this in subsequent patches. Hopefully this user flow will make it easy to review and track the changes made for this more general feature add.