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/batch.go

@@ -0,0 +1,34 @@
+package goleveldb
+
+import (
+	"github.com/syndtr/goleveldb/leveldb"
+	"github.com/syndtr/goleveldb/leveldb/opt"
+
+	"github.com/aptly-dev/aptly/database"
+)
+
+type batch struct {
+	db *leveldb.DB
+	b  *leveldb.Batch
+}
+
+func (b *batch) Put(key, value []byte) error {
+	b.b.Put(key, value)
+
+	return nil
+}
+
+func (b *batch) Delete(key []byte) error {
+	b.b.Delete(key)
+
+	return nil
+}
+
+func (b *batch) Write() error {
+	return b.db.Write(b.b, &opt.WriteOptions{})
+}
+
+// batch should implement database.Batch
+var (
+	_ database.Batch = &batch{}
+)