Choose fields for dynamic index

cc Asif Kazi Keshav Murthy

Allow users to choose fields / expressions for dynamic index. In addition to

PAIRS( SELF )

allow users to specify

PAIRS( { field1, field4, field9, ... } )

SELECT ... WHERE field1 > 5 AND ( field4 = "foo" OR field9 < 20 ) ...;

Only the chosen fields will be indexed, and queries will use the correct dynamic index.