Addendum patch fixing another false possitive FK constraint violation.

After the previous patch we ensure that the the SECONDARY_INDEX triggers
modifing the indexes are fired before the FK_CHILD / FK_PARENT
triggers which verifies the intergrity of the FK-constraints.

However, it turned out that the LQHKEYREQ(read) operations emitted
by the FK_CHILD / FK_PARENT triggers was optimized to read from
the local backup replicas if any such were found by TC. Thus, the
read could race an update/delete operation emitted by the preceeding
SECONDARY_INDEX trigger. Resulting in the local backup replica to
be read before the update/delete operation had been reflected in it.
This caused false FK-constraint violations to be reported.

This patch disables the 'read local replica' optimization if the
READ operation is part of a FK-constraint check.