rdbms: make selectRowCount() use $var argument to exclude NULLs

rdbms: make selectRowCount() use $var argument to exclude NULLs

If the $var argument is provided, then it will make the resulting
count exclude rows where the value for that column is NULL.

Also add buildSelectSubquery() method and Subquery
wrapper class for use with select() for calculated tables.

Change-Id: I549d629af99afdf370602de095f7fba6d1546c37