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/local_test.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 
 	"github.com/aptly-dev/aptly/database"
+	"github.com/aptly-dev/aptly/database/goleveldb"
 
 	. "gopkg.in/check.v1"
 )
@@ -18,7 +19,7 @@ type LocalRepoSuite struct {
 var _ = Suite(&LocalRepoSuite{})
 
 func (s *LocalRepoSuite) SetUpTest(c *C) {
-	s.db, _ = database.NewOpenDB(c.MkDir())
+	s.db, _ = goleveldb.NewOpenDB(c.MkDir())
 	s.list = NewPackageList()
 	s.list.Add(&Package{Name: "lib", Version: "1.7", Architecture: "i386"})
 	s.list.Add(&Package{Name: "app", Version: "1.9", Architecture: "amd64"})
@@ -83,7 +84,7 @@ type LocalRepoCollectionSuite struct {
 var _ = Suite(&LocalRepoCollectionSuite{})
 
 func (s *LocalRepoCollectionSuite) SetUpTest(c *C) {
-	s.db, _ = database.NewOpenDB(c.MkDir())
+	s.db, _ = goleveldb.NewOpenDB(c.MkDir())
 	s.collection = NewLocalRepoCollection(s.db)
 
 	s.list = NewPackageList()