Fix histograms for primary key of salted tables

Users attempting to use Update Statistics to create a multi-column
histogram (MC) corresponding to the primary key of a salted table may be
unaware that the "_SALT_" column is implicitly prepended to the key as
stated in the Create Table statement, and omit it. This fix will cause
Update Stats to detect a request for a multi-column histograms that
specifies the primary key columns (or a prefix of the full key), and
add _SALT_ to it if missing, and order the MC to match the order of the
columns in the primary key.

The change only affects salted tables, and is only applied if neither
the ON EVERY KEY nor ON EVERY COLUMN clauses is present, because an MC
matching the full primary key is automatically generated in those cases.

A second part of this fix applies to cases where ON EVERY KEY or ON
EVERY COLUMN is specified in an Update Statistics statement on a salted
table. By default, MCs corresponding to subsets of the primary key will
no longer be generated automatically in this case. The cqd
USTAT_ADD_SALTED_KEY_PREFIXES_FOR_MC may be set to 'ON' to cause MCs
for subsets of the primary key to be generated.

Closes-Bug #1336983

Change-Id: I930fef13371d5d773a0df44601cf99a8a4dc8322
Merge "Fix histograms for primary key of salted tables"