Implemented FetchAssetMetadataSet in DB backends.

This method fetches metadata for a subset of the entries in the assets
database. This functionality is used in the ForEach calls in the asset
storage providers in AssetInventoryServer. With this implemented,
frontends such as the BrowseFrontend should now work.

- MySQL: implemented, sanity tested
- SQLite: implemented, sanity tested
- MSSQL: implemented, not tested
- NHibernate: not implemented

OpenSim/Tests/Common/Mock/TestAssetDataPlugin.cs

@@ -25,6 +25,7 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+using System.Collections.Generic;
 using OpenMetaverse;
 using OpenSim.Framework;
 using OpenSim.Data;
@@ -48,6 +49,7 @@ namespace OpenSim.Tests.Common.Mock
         public void CreateAsset(AssetBase asset) {}
         public void UpdateAsset(AssetBase asset) {}
         public bool ExistsAsset(UUID uuid) { return false; }
+        public List<AssetMetadata> FetchAssetMetadataSet(int start, int count) { return new List<AssetMetadata>(count); }
         public void Initialise(string connect) {}
     }
 }