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

merkle/simple_map.go

@@ -0,0 +1,26 @@
+package merkle
+
+type SimpleMap struct {
+	kvz KVPairs
+}
+
+func NewSimpleMap() *SimpleMap {
+	return &SimpleMap{
+		kvz: nil,
+	}
+}
+
+func (sm *SimpleMap) Set(k string, o interface{}) {
+	sm.kvz = append(sm.kvz, KVPair{Key: k, Value: o})
+}
+
+// Merkle root hash of items sorted by key.
+// NOTE: Behavior is undefined when key is duplicate.
+func (sm *SimpleMap) Hash() []byte {
+	sm.kvz.Sort()
+	kvPairsH := make([]Hashable, 0, len(sm.kvz))
+	for _, kvp := range sm.kvz {
+		kvPairsH = append(kvPairsH, kvp)
+	}
+	return SimpleHashFromHashables(kvPairsH)
+}