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.

database/goleveldb/transaction.go

@@ -0,0 +1,60 @@
+package goleveldb
+
+import (
+	"bytes"
+
+	"github.com/aptly-dev/aptly/database"
+	"github.com/syndtr/goleveldb/leveldb"
+)
+
+type transaction struct {
+	t *leveldb.Transaction
+}
+
+// Get implements database.Reader interface.
+func (t *transaction) Get(key []byte) ([]byte, error) {
+	value, err := t.t.Get(key, nil)
+	if err != nil {
+		if err == leveldb.ErrNotFound {
+			return nil, database.ErrNotFound
+		}
+		return nil, err
+	}
+
+	return value, nil
+}
+
+// Put implements database.Writer interface.
+func (t *transaction) Put(key, value []byte) error {
+	old, err := t.t.Get(key, nil)
+	if err != nil {
+		if err != leveldb.ErrNotFound {
+			return err
+		}
+	} else {
+		if bytes.Equal(old, value) {
+			return nil
+		}
+	}
+	return t.t.Put(key, value, nil)
+}
+
+// Delete implements database.Writer interface.
+func (t *transaction) Delete(key []byte) error {
+	return t.t.Delete(key, nil)
+}
+
+// Commit finalizes transaction and commits changes to the stable storage.
+func (t *transaction) Commit() error {
+	return t.t.Commit()
+}
+
+// Discard any transaction changes.
+//
+// Discard is safe to call after Commit(), it would be no-op
+func (t *transaction) Discard() {
+	t.t.Discard()
+}
+
+// transaction should implement database.Transaction
+var _ database.Transaction = &transaction{}