Clear branch cache on full reconfiguration

The full reconfiguration event is primarily used to correct invalid
states which may result from missed events, such as a branch creation.
However, it did not invalidate or update the actual data in the branch
cache.  Correct this by completely clearing the branch cache of every
connection when starting a full reconfiguration.  This will cause the
connections to query their upstream sources the next time that we ask
for a list of branches.

Change-Id: I1529fe1dd6092de612a94c14ec0527ab76f62e47

tests/fixtures/config/one-project/git/common-config/playbooks/base.yaml

@@ -0,0 +1,2 @@
+- hosts: all
+  tasks: []

tests/fixtures/config/one-project/git/common-config/zuul.yaml

@@ -0,0 +1,41 @@
+- pipeline:
+    name: check
+    manager: independent
+    trigger:
+      gerrit:
+        - event: patchset-created
+    success:
+      gerrit:
+        Verified: 1
+    failure:
+      gerrit:
+        Verified: -1
+
+- pipeline:
+    name: gate
+    manager: dependent
+    success-message: Build succeeded (gate).
+    trigger:
+      gerrit:
+        - event: comment-added
+          approval:
+            - Approved: 1
+    success:
+      gerrit:
+        Verified: 2
+        submit: true
+    failure:
+      gerrit:
+        Verified: -2
+    start:
+      gerrit:
+        Verified: 0
+    precedence: high
+
+- job:
+    name: base
+    parent: null
+    run: playbooks/base.yaml
+
+- job:
+    name: test-job

tests/fixtures/config/one-project/git/org_project/zuul.yaml

@@ -0,0 +1,8 @@
+- project:
+    name: org/project
+    check:
+      jobs:
+        - test-job
+    gate:
+      jobs:
+        - test-job

tests/fixtures/config/one-project/main.yaml

@@ -0,0 +1,9 @@
+- tenant:
+    name: tenant-one
+    source:
+      gerrit:
+        config-projects:
+          - common-config
+        untrusted-projects:
+          - org/project
+