Concurrency Fixes for RxRingBuffer & Merge

Possible fix for #1420

I was unable to exactly reproduce the issue, but was able to create a possibly related one. This fixes it.
It also simplifies the logic and code somewhat, at a possible small performance cost for these:

```
r.o.OperatorMergePerf.merge1SyncStreamOfN            1000  thrpt         5    59407.716     2073.945    ops/s
r.o.OperatorMergePerf.merge1SyncStreamOfN         1000000  thrpt         5       66.557        2.693    ops/s
r.o.OperatorMergePerf.mergeNSyncStreamsOfN           1000  thrpt         5       59.293        5.046    ops/s
```