This fix makes two basic changes in blob handling:
(The bug was introduced in r2252)

1) The blob prefixes are no longer stored in the undo if
  a) We are modifying a delete marked record and
  b) The record was delete marked by an already committed trx.

2) When building old row version to check if one of these versions
can hold an implicit lock on the record we stop our probe if
  a) The version is delete marked and
  b) The delete marking is done by a trx which is different from
     the current active trx on the record.

Reviewed by: Heikki