Don't cache github libraries above nginx server.

It's too expensive to fetch the timestamps when the github
server is busy.  See Dick Hollenbeck's comments at the top of
github_plugin.cpp for more info.

Also adds some safety to the other caching algorithms after
seeing github_plugin's wild-west usage of the kicad_plugin.

Fixes: lp:1753143
* https://bugs.launchpad.net/kicad/+bug/1753143

common/fp_lib_table.cpp

@@ -241,7 +241,7 @@ long long FP_LIB_TABLE::GenerateTimestamp( const wxString* aNickname )
     {
         const FP_LIB_TABLE_ROW* row = FindRow( *aNickname );
         wxASSERT( (PLUGIN*) row->plugin );
-        return row->plugin->GetLibraryTimestamp();
+        return row->plugin->GetLibraryTimestamp( row->GetFullURI( true ) );
     }
 
     long long hash = 0;
@@ -250,7 +250,7 @@ long long FP_LIB_TABLE::GenerateTimestamp( const wxString* aNickname )
     {
         const FP_LIB_TABLE_ROW* row = FindRow( nickname );
         wxASSERT( (PLUGIN*) row->plugin );
-        hash += row->plugin->GetLibraryTimestamp();
+        hash += row->plugin->GetLibraryTimestamp( row->GetFullURI( true ) );
     }
 
     return hash;