sctp: increase DEFAULT_NUMBER_OF_SCTP_STREAMS to 1024
This change makes it possible to create more than just 5 webrtc data channels. The maximum number of data channels is exactly DEFAULT_NUMBER_OF_SCTP_STREAMS / 2, therefore the limit is now 512.
This fixes issue #954 (closed).
1024 is the value set by WebKit (see WebKit's sctptransportinternal.h:41 and sctptransport.cc:213)