WIP Moving back towards using named runs.

* Rename artifactsRepo -> buildDataDir to be more explicit about the fact that
  it holds more than just the artifacts.
* Revert removal of run ids.
* Move Worker definition into core as part of making the core responsible for
  accepting run requests.
* Make the core module more responsible for internal details of data structure
  and storage. External callers should not need to construct paths to
  artifacts, versions, etc. but should be able to call method in the core
  module to do this work for them.
* The working directory no longer contains anything but the checked-out code.
  All StrawBoss-specific data is stored by StrawBoss elsewhere.
* Add a regular maintenance cycle to the server module.

file-structure.txt

@@ -0,0 +1,17 @@
+build-data/
+  <project-name>/
+    configurations/
+      <version>.json
+    runs/
+      <id>.request.json
+      <id>.stdout.log
+      <id>.stderr.log
+      <id>.status.json
+    status/
+      <version>.json
+    artifacts/
+      <step-name>/
+        <version>/
+          <artifact-file>
+
+workspace/