+ make copies of the []bytes returned by goleveldb

- The byte strings returned by goleveldb aren't necessarily safe.  See
    the following google group thread:

    https://groups.google.com/forum/#!topic/bleve/aHZ8gmihLiY

    This code change is based on the gist created here:

    https://groups.google.com/forum/#!topic/bleve/aHZ8gmihLiY

index/store/goleveldb/iterator.go

@@ -59,11 +59,17 @@ func (ldi *Iterator) Current() ([]byte, []byte, bool) {
 }
 
 func (ldi *Iterator) Key() []byte {
-	return ldi.iterator.Key()
+	k := ldi.iterator.Key()
+	rv := make([]byte, len(k))
+	copy(rv, k)
+	return rv
 }
 
 func (ldi *Iterator) Value() []byte {
-	return ldi.iterator.Value()
+	v := ldi.iterator.Value()
+	rv := make([]byte, len(v))
+	copy(rv, v)
+	return rv
 }
 
 func (ldi *Iterator) Valid() bool {

index/store/goleveldb/store.go

@@ -52,7 +52,9 @@ func (ldbs *Store) get(key []byte) ([]byte, error) {
 	if err == leveldb.ErrNotFound {
 		return nil, nil
 	}
-	return b, err
+	rv := make([]byte, len(b))
+	copy(rv, b)
+	return rv, err
 }
 
 func (ldbs *Store) getWithSnapshot(key []byte, snapshot *leveldb.Snapshot) ([]byte, error) {
@@ -61,7 +63,9 @@ func (ldbs *Store) getWithSnapshot(key []byte, snapshot *leveldb.Snapshot) ([]by
 	if err == leveldb.ErrNotFound {
 		return nil, nil
 	}
-	return b, err
+	rv := make([]byte, len(b))
+	copy(rv, b)
+	return rv, err
 }
 
 func (ldbs *Store) set(key, val []byte) error {