Changes

erab_fsm: add erab_release/2 and erab_release_ind/1

The E-RAB RELEASE INDICATION message is used by the eNB to indicate
termination of one or several E-RAB(s).  Unlike with the RELEASE
COMMAND, there is no response to that message.  Thus, we shall not
wait for it (by entering state erab_wait_release_rsp) but terminate
the E-RAB FSM immediately instead.

Change-Id: Icc9c0f6a1aef327e9585942b3c9d2302feda393a