Checks the constraint against the current row and the current modification.
The existingRow entity is guaranteed to be a row which exists (e.g. there are KeyValues there) and
is visible to the transaction that this constraint is to be applied within.
For example, suppose you are checking a constraint on row A with transaction t.
If row A does not exist yet in HBase, this method will not be called and the constraint will not be checked.
If row A already exists, but was written with a conflicting transaction, this method will not be checked
(because a Write/Write conflict will be thrown).
If row A exists, but the row is not visible to the current transaction (i.e. it was rolled back), then
this method will not be checked.
If row A exists, and the row is visible to transaction t, this method will be called.