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/util.go

@@ -0,0 +1,82 @@
+package db
+
+import "bytes"
+
+// A wrapper around itr that tries to keep the iterator
+// within the bounds as defined by `prefix`
+type prefixIterator struct {
+	itr     Iterator
+	prefix  []byte
+	invalid bool
+}
+
+func (pi *prefixIterator) Seek(key []byte) {
+	if !bytes.HasPrefix(key, pi.prefix) {
+		pi.invalid = true
+		return
+	}
+	pi.itr.Seek(key)
+	pi.checkInvalid()
+}
+
+func (pi *prefixIterator) checkInvalid() {
+	if !pi.itr.Valid() {
+		pi.invalid = true
+	}
+}
+
+func (pi *prefixIterator) Valid() bool {
+	if pi.invalid {
+		return false
+	}
+	key := pi.itr.Key()
+	ok := bytes.HasPrefix(key, pi.prefix)
+	if !ok {
+		pi.invalid = true
+		return false
+	}
+	return true
+}
+
+func (pi *prefixIterator) Next() {
+	if pi.invalid {
+		panic("prefixIterator Next() called when invalid")
+	}
+	pi.itr.Next()
+	pi.checkInvalid()
+}
+
+func (pi *prefixIterator) Prev() {
+	if pi.invalid {
+		panic("prefixIterator Prev() called when invalid")
+	}
+	pi.itr.Prev()
+	pi.checkInvalid()
+}
+
+func (pi *prefixIterator) Key() []byte {
+	if pi.invalid {
+		panic("prefixIterator Key() called when invalid")
+	}
+	return pi.itr.Key()
+}
+
+func (pi *prefixIterator) Value() []byte {
+	if pi.invalid {
+		panic("prefixIterator Value() called when invalid")
+	}
+	return pi.itr.Value()
+}
+
+func (pi *prefixIterator) Close()          { pi.itr.Close() }
+func (pi *prefixIterator) GetError() error { return pi.itr.GetError() }
+
+func IteratePrefix(db DB, prefix []byte) Iterator {
+	itr := db.Iterator()
+	pi := &prefixIterator{
+		itr:    itr,
+		prefix: prefix,
+	}
+	pi.Seek(prefix)
+	return pi
+}