Jae/fixprefixdb (#199)

* Fix PrefixDB Iterator
* PrefixDB Iterator/ReverseIterator fixes
* Bump version 0.8.2
* Update CHANGELOG.md about DebugDB
* Keep invalid source to be closed
* Use prefixBatch instead of memBatch

db/mem_batch.go

@@ -1,6 +1,8 @@
 package db
 
-import "sync"
+import (
+	"sync"
+)
 
 type atomicSetDeleter interface {
 	Mutex() *sync.Mutex
@@ -66,6 +68,5 @@ func (mBatch *memBatch) write(doSync bool) {
 		case opTypeDelete:
 			mBatch.db.DeleteNoLock(op.key)
 		}
-
 	}
 }