CacheDB (#67)

* Add CacheDB & SimpleMap
* Generic memBatch; Fix cLevelDB tests
* CacheWrap() for CacheDB and MemDB
* Change Iterator to match LeviGo Iterator
* Fixes from review
* cacheWrapWriteMutex and some race fixes
* Use tmlibs/common
* NewCWWMutex is exposed.  DB can be CacheWrap'd
* Remove GetOK, not needed
* Fsdb (#72)
* Add FSDB
* Review fixes from Anton
* Review changes
* Fixes from review

db/backend_test.go

@@ -0,0 +1,43 @@
+package db
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	cmn "github.com/tendermint/tmlibs/common"
+)
+
+func testBackend(t *testing.T, backend string) {
+	// Default
+	dir, dirname := cmn.Tempdir(fmt.Sprintf("test_backend_%s_", backend))
+	defer dir.Close()
+	db := NewDB("testdb", backend, dirname)
+	require.Nil(t, db.Get([]byte("")))
+	require.Nil(t, db.Get(nil))
+
+	// Set empty ("")
+	db.Set([]byte(""), []byte(""))
+	require.NotNil(t, db.Get([]byte("")))
+	require.NotNil(t, db.Get(nil))
+	require.Empty(t, db.Get([]byte("")))
+	require.Empty(t, db.Get(nil))
+
+	// Set empty (nil)
+	db.Set([]byte(""), nil)
+	require.NotNil(t, db.Get([]byte("")))
+	require.NotNil(t, db.Get(nil))
+	require.Empty(t, db.Get([]byte("")))
+	require.Empty(t, db.Get(nil))
+
+	// Delete
+	db.Delete([]byte(""))
+	require.Nil(t, db.Get([]byte("")))
+	require.Nil(t, db.Get(nil))
+}
+
+func TestBackends(t *testing.T) {
+	testBackend(t, CLevelDBBackendStr)
+	testBackend(t, GoLevelDBBackendStr)
+	testBackend(t, MemDBBackendStr)
+}