Keep track of which attachments are referenced by which sync entries.

sync/protocol/attachments.proto

Index: sync/protocol/attachments.proto
diff --git a/sync/protocol/attachments.proto b/sync/protocol/attachments.proto
index 50815d3d044829023d4dcb72930e4b285dfffba6..e08e2b052ceb73293751e4ec11a75a5869224fb3 100644
--- a/sync/protocol/attachments.proto
+++ b/sync/protocol/attachments.proto
@@ -21,7 +21,16 @@ message AttachmentIdProto {
   optional string unique_id = 1;
 }
 
+// Metadata for a single attachment.
+message AttachmentMetadataRecord {
+  optional AttachmentIdProto id = 1;
+  // Indicates we know this attachment exists on the server.
+  optional bool is_on_server = 2;
+}
+
 // A collection of attachment metadata. This proto is part of EntryKernel's "on
 // disk" representation. Private to sync.
 message AttachmentMetadata {
+  // One record per attachment.
+  repeated AttachmentMetadataRecord record = 1;
 }