Changes

sctp_server: make it abstract from sctp_proxy logic

Instead of using the sctp_proxy API directly, let the caller pass
a module name (handler), which implements the sctp_client behavior.

This makes the interaction between modules cleaner, allows to reuse
the sctp_server module in other places, and makes testing easier.

Change-Id: Iee4202dd4d4e9762fe74ead985aa43343ebdc524