added support for externalized Sequence numbers.

  -- sql statements: create/drop/alter/get/showddl sequence for sequence objects
  -- function 'seqnum' to retrieve sequence numbers.
An external spec has been created.
Launchpad #1349985
  Code reviewd by Joanie C, Suresh S, Selva and Sandhya.
  Full dev regressions run and passed.

Change-Id: Ie11dbab4d24ff6a1106697f7e2253ea895e6c873
Merge "added support for externalized Sequence numbers."