Allow tools to output collections with a dynamic number of datasets.

Models:

Track whether dataset collections have been populated yet.

Dataset collections are still effectively immutable once populated - but dynamic output collections require them to be sort of likely `final` fields in Java (analogy courtesy of JJ) - allowing them to be declared before they are initialized or populated. This is tracked by the `populated_state` field.

Tools:

Output collections can now describe `discover_datasets` elements just like datasets - except in this case instead of dynamically populating new datasets in the history - they will comprise the collection. `designation` has been reused to serve as the element_identifier for the collection element corresponding to the dataset.

See Pull Request 356 for more information on the discover_datasets tag https://bitbucket.org/galaxy/galaxy-central/pull-request/356/enhancements-for-runtime-discovered.

Workflows:

Update workflow execution and recovery for dynamic output collections.

Galaxy workflow data flow before collections

* - * - * - * - * - *

Galaxy worfklow data flow after collections (iteration 1)

* - * - * \
           * - * - *
* - * - * /         \
                     * - * - *
* - * - * \         /
           * - * - *
* - * - * /

Galaxy worfklow data flow after this commit

              / * - * \
         * - *         * - *
        /     \ * - * /     \
       /                     \
      /                       \
     /        / * - * \        \
* - * -- * - *         * - * -- * - *
     \        \ * - * /        /
      \                       /
       \                     /
        \     / * - * \     /
         * - *         * - *
              \ * - * /
Implement testing of output collections and elements.
Prevent tools producing collections from showing in workflow editor.

Certainly feasible someday but workflow editor logic will require significant overhaul.
Revise state handling to handle errors while populating state.
Improve structure of output_collect stuff.
More work on collections.