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.

context/context.go

@@ -17,6 +17,7 @@ import (
 	"github.com/aptly-dev/aptly/aptly"
 	"github.com/aptly-dev/aptly/console"
 	"github.com/aptly-dev/aptly/database"
+	"github.com/aptly-dev/aptly/database/goleveldb"
 	"github.com/aptly-dev/aptly/deb"
 	"github.com/aptly-dev/aptly/files"
 	"github.com/aptly-dev/aptly/http"
@@ -252,7 +253,7 @@ func (context *AptlyContext) _database() (database.Storage, error) {
 	if context.database == nil {
 		var err error
 
-		context.database, err = database.NewDB(context.dbPath())
+		context.database, err = goleveldb.NewDB(context.dbPath())
 		if err != nil {
 			return nil, fmt.Errorf("can't instantiate database: %s", err)
 		}