Submgr REST subscriptions in SDL Database

   * renamed sdl.go  to sdl_e2SubsDb.go
   * renamed sdl_test.go to sdl_e2SubsDb_test.go
   * New file sdl_restSubsDb.go implements the SDL API for the REST Subscriptions
   * New file sdl_restSubsDb_test.go implements unittests for the SDL API for the REST Subscriptions
   * New db restSubsDb defined under Control object in submgr to hold the SDL DB data structure
   * SDL API for the REST Subscriptions is called from submgr code.

Signed-off-by: Konstantinos Archangelof <konstantinos.archangelof@nokia.com>
Change-Id: I447da9030aba2a582e38d24ef86722894d0c41e1