Refactor database code to support standalone batches, transactions.

This is spin-off of changes from #459.

Transactions are not being used yet, but batches are updated to work
with the new API.

`database/` package was refactored to split abstract interfaces and
implementation via goleveldb. This should make it easier to implement
new database types.

cmd/db_recover.go

@@ -1,8 +1,9 @@
 package cmd
 
 import (
-	"github.com/aptly-dev/aptly/database"
 	"github.com/smira/commander"
+
+	"github.com/aptly-dev/aptly/database/goleveldb"
 )
 
 // aptly db recover
@@ -15,7 +16,7 @@ func aptlyDbRecover(cmd *commander.Command, args []string) error {
 	}
 
 	context.Progress().Printf("Recovering database...\n")
-	err = database.RecoverDB(context.DBPath())
+	err = goleveldb.RecoverDB(context.DBPath())
 
 	return err
 }