rosie.ws_client: support multiple sources

`rosie.ws_client` is modified to work with multiple prefixes.
Use `multiprocessing.Pool` to send requests to the sources in parallel.
This requires a pickle-safe version of python-requests.

The `--prefix=PREFIX` for `rosie go`, `rosie lookup` and `rosie ls` can
now be specified multiple times. The lookup mode is rationalised into a
single option, with aliases.

`rosie go` updated to allow users to select any data source combination.

`rosie.suite_id`: The logic to obtain other user's local copy root is
not ideal, but there is no easy way to make it more general, so accept
that we hard code at the moment.

Tests:
* t/rosie-ls: new basic tests.
* t/rosie-lookup: new basic multi sources tests.

Other changes:
* Refactor.
* Pass `pep8` for modified modules.
Update version requirement for python-requests
rosie refactor: update date format, doc, etc
Fix tests
rose.config_tree: improve pep8 and pylint scores
rose.config_tree: store file lists

If a relative path can be found in 2 or more locations in the linearised
inheritance, it will store the configuration locations in a new list.
Use rose.config_tree to load metadata

Part 1.
rose.macros.trigger: use rose.config_tree

to load metadata.
Merge branch 'master' into refactor-rosie.ws_client
rose edit: um stash widget: record help change
revert style changes
Merge branch 'master' into refactor-rosie.ws_client
rosie refactor: various syntax improvements
Fix syntax in a test
rosie db: fix failure for no-opt-props suite.

This fixes a search or query processing error when the results contain
a suite with no optional properties at all (i.e. just owner, project,
and title).
Doc: fix ROSE_DATAC???? syntax
Merge pull request #1406 from benfitzpatrick/um.stash-help-use-stash-codes

rose edit: UM stash widget: stash record help input change
Merge pull request #1409 from benfitzpatrick/rosie-db.fix-no-optional-props

rosie db: fix failure for no-opt-props suite.
Merge branch 'master' into refactor-rosie.ws_client

Conflicts:
	t/rosie-lookup/00-basic.t
Merge pull request #1411 from matthewrmshin/fix-doc-datac-time

Doc: fix ROSE_DATAC???? syntax