Problem:

When a unique secondary index is scanned for duplicate checking, gap locks
were not taken if the transaction had isolation level <= READ COMMITTED. 
This change was done while fixing Bug #16133801 UNEXPLAINABLE INNODB UNIQUE
INDEX LOCKS ON DELETE + INSERT WITH SAME VALUES (rb#2035). Because of this
the duplicate check logic failed, and resulted in duplicate values in unique
secondary index.

Solution:

When a unique secondary index is scanned for duplicate checking, gap locks
must be taken irrespective of the transaction isolation level.  This is
achieved by reverting rb#2035.

rb#5910 approved by Jimmy