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.

deb/changes_test.go

@@ -7,6 +7,7 @@ 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/files"
 	"github.com/aptly-dev/aptly/utils"
 
@@ -37,7 +38,7 @@ func (s *ChangesSuite) SetUpTest(c *C) {
 	err := utils.CopyFile("testdata/changes/calamares.changes", s.Path)
 	c.Assert(err, IsNil)
 
-	s.db, _ = database.NewOpenDB(c.MkDir())
+	s.db, _ = goleveldb.NewOpenDB(c.MkDir())
 	s.localRepoCollection = NewLocalRepoCollection(s.db)
 	s.packageCollection = NewPackageCollection(s.db)