Support enqueuing behind circular dependencies

Change Ic121b2d8d057a7dc4448ae70045853347f265c6c omitted support
for enqueuing changes behind a cycle in order to avoid having them
show up in the middle of the cycle.  However, we can process changes
behind a cycle if we wait until we have completely enqueued the
cycle before we embark on enqueing changes behind it.

This also removes unused pipelines which cause extra processing and
output in the unit tests.

Also add a release note about circular deps, and describe additional
caveats about using circular dependencies in the documentation.

Change-Id: I64c0d4e8c20e4638bbafb18409cd28c062369738

tests/fixtures/config/circular-dependencies/git/common-config/zuul.yaml

@@ -26,23 +26,6 @@
       github:
         status: failure
 
-- pipeline:
-    name: check-unused
-    manager: independent
-    trigger:
-      gerrit:
-        - event: patchset-created
-    success:
-      gerrit:
-        Verified: 1
-      github:
-        status: success
-    failure:
-      gerrit:
-        Verified: -1
-      github:
-        status: failure
-
 - pipeline:
     name: gate
     manager: dependent
@@ -80,31 +63,6 @@
       github: {}
     precedence: high
 
-- pipeline:
-    name: gate-unused
-    manager: dependent
-    success-message: Build succeeded (gate).
-    trigger:
-      gerrit:
-        - event: comment-added
-          approval:
-            - Approved: 1
-    success:
-      gerrit:
-        Verified: 2
-        submit: true
-      github:
-        merge: true
-    failure:
-      gerrit:
-        Verified: -2
-      github: {}
-    start:
-      gerrit:
-        Verified: 0
-      github: {}
-    precedence: high
-
 - job:
     name: base
     parent: null