Problem:

The following problems have been identified:

1. If the row format is REDUNDANT or COMPACT (both of them have blob
   prefix) and transaction isolation level is READ UNCOMMITTED, then it
   is possible to return only the local prefix of the LOB.  In the case
   of JSON documents, this will be seen as a corrupted document.

2. If the row format is REDUNDANT or COMPACT (both of them have blob
   prefix) and transaction isolation level is READ UNCOMMITTED, then it
   is possible to return the local prefix of the new LOB + external
   part of the old blob.  In the case of JSON documents, this will be
   seen as a corrupted document.

3. If the row format is DYNAMIC (no blob prefix) and transaction isolation
   level is READ UNCOMMITTED, then it is possible to return old LOB
   with new value of other fields. While no JSON document will be
   considered corrupted, it will be inconsistent to return old value
   for blob column and new value for another column.

Solution:

The solution consists of 2 parts:

1. Either send the full document or none.  Don't send only the local
   prefix.

2. Before releasing latches, mark the blobs as 'being modified' in the
   blob reference object stored in the clust_rec.

rb#19446 approved by Bin Su.