Change index merging to only sort by keyed fields

Currently the key merging code compares all fields when merging rows from multiple sources.  It currently does this using a memcmp on the maximum length of the key record.  This has several problems:

• It could access uninitialised data.
• The payload may not be sorted in a memcmp order - so may return rows out of order
• If field translation is in use there is no guarantee that the rows will be ordered

I propose changing the comparison so that it only compared the key fields.  If they are identical the row from the lower part is returned.

I think the key merger is currently used in the following situations:

a) Smart stepping.  This should be unaffected by the change.

b) Merging results from local indexes.  This will potentially change.

c) Keyed join in thor.  I suspect this is for convenience.  For global keys it may be more efficient to use a key manager that does not compare - and takes rows from parts in order.

jakesmith richardkchapman