Various LP fixes, bugs and code cleanup.

-- removed obsolete code (label create/alter/delete, get disk/label/buffer stats,
dp2 scan)
-- metadata structs are now created as classes and initialized during
creation.  LP 1394649
-- warnings are now being returned from compiler to executor after DDL operations.
-- duplicate constraint names now return error.
-- handle NOT ENFORCED constraints: give warning during creation and not enforce
during use. LP 1361784
-- drop all indexes (enabled and disabled indexes) on a table during drop table
and schema now works.  LP 1384380
-- drop constraint on disabled index succeeds. LP 1384479
-- string truncation error is now returned if default value doesn't fit in
column. LP 1394780
-- fixed issue where a failure during multiple constraints creation in a create
stmt was not cleaning up metadata. LP 1389871
-- update where current of is now supported. LP 1324679

Change-Id: Iec1b0b4fc6a8161a33b7f69228c0f1e3f441f330