server: Auto-populate liza_timestamp_initial_rated in metabucket

This value already existed on the document, but was inaccessible to external
systems.  This is now accessible to e.g. raters.

* doc/bucket.texi (Metabucket): New section.
* src/server/db/MongoServerDao.js (saveQuote): Set initial quoted date as
    liza_timestamp_initial_rated.  Update metabucket keys individually so as
    not to inadvertently overwrite the entire metabucket.

DEV-3715

doc/bucket.texi

@@ -16,6 +16,7 @@
 * Value Assignment:Bucket Assignment.  Writing data to the Bucket.
 * Bucket Diff::                        Representing bucket changes.
 * Calculated Values::                  Dynamic data derived from other values.
+* Metabucket::                         Bucket holding document metadata
 @end menu
 
 
@@ -116,3 +117,32 @@ A diff is applied to the underlying bucket by invoking
 @node Calculated Values
 @section Calculated Values
 @helpwanted
+
+
+@node Metabucket
+@section Metabucket
+@cindex Metabucket
+The @dfn{metabucket} is a loosely-structured key/value store
+  separate from the data bucket.@footnote{
+    It is stored in the @code{meta} field on the Mongo document.}
+It should be used to save data that should be accessible only to the server,
+  but never the client.
+
+The client has no means by which to access the metabucket.
+Custom fields can be populated by server-side DataAPIs
+  (@pxref{Server-Side Data API Calls}).
+
+Any fields prefixed with the string @samp{liza_} are reserved and are
+  populated automatically by the Server.
+They are shown in @ref{t:liza-meta}.
+
+@float Table, t:liza-meta
+@table @code
+  @cindex Initial rated date
+  @item liza_timestamp_initial_rated
+  A Unix timestamp representing the first time a document was acted
+    upon by a rating service.
+  This value is set once and is never updated or cleared.
+@end table
+@caption{Metabucket fields populated automatically by the Server}
+@end float

src/server/db/MongoServerDao.js

@@ -287,7 +287,8 @@ module.exports = Class( 'MongoServerDao' )
         quote, success_callback, failure_callback, save_data
     )
     {
-        var dao = this;
+        var   dao = this;
+        const meta = {};
 
         // if we're not ready, then we can't save the quote!
         if ( this._ready === false )
@@ -309,7 +310,7 @@ module.exports = Class( 'MongoServerDao' )
             };
 
             // full save will include all metadata
-            save_data.meta = quote.getMetabucket().getData();
+            meta = quote.getMetabucket().getData();
         }
 
         // when we update the quote data, clear quick save data (this data
@@ -333,9 +334,17 @@ module.exports = Class( 'MongoServerDao' )
             ( new Date() ).getTime() / 1000
         );
 
+        // meta will eventually take over for much of the above data
+        meta.liza_timestamp_initial_rated = quote.getRatedDate();
+
         // save the stack so we can track this call via the oplog
         save_data._stack = ( new Error() ).stack;
 
+        // avoid wiping out other metadata (since this may not be a full set)
+        Object.keys( meta ).forEach(
+            key => save_data[ 'meta.' + key ] = meta[ key ]
+        );
+
         // update the quote data if it already exists (same id), otherwise
         // insert it
         this._collection.update( { id: id },