Split the groups external cache into 2 caches for better performance

The current implementation of the groups external cache used an
in-memory cache with a single key "EXTERNAL_NAME" that maps to the list
of all external groups.

In this change we almost left this cache intact, but modified its loader
to retrieve the value (external groups) using a persisted external
groups cache. The persistent cache has a key that represents the state
of NoteDb, and specifically is the concatenation of the SHA-1 of all
internal groups' refs in NoteDb. The persistent cache's loader does the
expensive operation of loading the groups from NoteDb.

Change-Id: Iac48415589eabec479dc658eaa20c2dd4c7b4932

proto/cache.proto

diff --git a/proto/cache.proto b/proto/cache.proto
index 5fc5e68..e0c4400 100644
--- a/proto/cache.proto
+++ b/proto/cache.proto
@@ -270,6 +270,15 @@
   repeated ExternalIdProto external_id = 1;
 }
 
+// Serialized form of a list of com.google.gerrit.entities.AccountGroup.UUID
+// Next ID: 2
+message AllExternalGroupsProto {
+  message ExternalGroupProto {
+    string groupUuid = 1;
+  }
+  repeated ExternalGroupProto external_group = 1;
+}
+
 // Key for com.google.gerrit.server.git.PureRevertCache.
 // Next ID: 4
 message PureRevertKeyProto {