mem: Initial in-memory store implementation for #88

- Reduce default retention sleep, change description.

pkg/storage/mem/store_test.go

@@ -0,0 +1,17 @@
+package mem
+
+import (
+	"testing"
+
+	"github.com/jhillyerd/inbucket/pkg/storage"
+	"github.com/jhillyerd/inbucket/pkg/test"
+)
+
+// TestSuite runs storage package test suite on file store.
+func TestSuite(t *testing.T) {
+	test.StoreSuite(t, func() (storage.Store, func(), error) {
+		s := New()
+		destroy := func() {}
+		return s, destroy, nil
+	})
+}