Merge branch 'maint-0.3.1'

changes/bug22883-priority

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+  o Major bugfixes (relay, performance):
+
+    - Perform circuit handshake operations at a higher priority than we use
+      for consensus diff creation and compression. This should prevent
+      circuits from starving when a relay or bridge receive a new consensus,
+      especially on lower-powered machines. Fixes bug 22883; bugfix on
+      0.3.1.1-alpha.
+

changes/more-threads

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+  o Minor features (relay, performance):
+    - Always start relays with at least two worker threads, to prevent
+      priority inversion on slow tasks.  Part of the fix for bug 22883.

changes/multi-priority

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+  o Minor features (relay, thread pool):
+    - Allow background work to be queued with different priorities, so
+      that a big pile of slow low-priority jobs will not starve out
+      higher priority jobs. This lays the groundwork for a fix for bug
+      22883.