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/Data/AssetDataBase.cs

@@ -25,6 +25,7 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+using System.Collections.Generic;
 using OpenMetaverse;
 using OpenSim.Framework;
 
@@ -37,6 +38,8 @@ namespace OpenSim.Data
         public abstract void UpdateAsset(AssetBase asset);
         public abstract bool ExistsAsset(UUID uuid);
 
+        public abstract List<AssetMetadata> FetchAssetMetadataSet(int start, int count);
+
         public abstract string Version { get; }
         public abstract string Name { get; }
         public abstract void Initialise(string connect);