Changes

sctp_common: gen_sockopts/1: add heartbeat and RTO params

Set the SCTP heartbeat interval and Retransmission time-out params
explicitly for both sctp_{client,server}.  Make the relevant values
configurable via the environment (config file).

Change-Id: I081852d2df187ca7c3cb8969686530b8ac3644f3
Co-authored-by: Andreas Eversberg <jolly@eversberg.eu>
Related: SYS#7653