Fix small bug in collector.c
- this appeared while fixing GC bug
- the sync rec should be sent only when the ready stream set is empty --> otherwise, it may forward unexpected sort end. This happens when there is initial branches mixed with non-initial ones.
Fix bug in garbage collector + add SNetStreamGetId(for debugging)
 - Fix bug: "Received sort records do not match"
    This happens as the sort_end records do not pass the parallel entity
    but pass the collector entity (See the gc report for more detail)
    This is fixed by sending a special sort_end record to notify the
    paired collector when a parallel entity terminates itself.
Forgot to add the SNetStreamGetId declaration
Merge branch 'master' into centralQ
 - to fix the bug of garbage collector