Migrate from spsc to spmc

I'm seeing occasional failures that I tracked down to concurrency issues by using an spsc queue. The consumer thread can change due to how merge works, but I thought the emitLock would be a sufficient enough barrier that it would behave as a "single consumer" but apparently that's not the case. This changes it to be a "multi consumer" queue.

Perf tests suggest the impact of this change is minor if at all noticeable:

```

Benchmark                                          (size)   Mode   Samples        Score  Score error    Units
r.o.OperatorMergePerf.merge1SyncStreamOfN               1  thrpt         5  5346594.021    93344.077    ops/s
r.o.OperatorMergePerf.merge1SyncStreamOfN            1000  thrpt         5    83109.197     4405.491    ops/s
r.o.OperatorMergePerf.merge1SyncStreamOfN         1000000  thrpt         5       71.645        2.918    ops/s
r.o.OperatorMergePerf.mergeNAsyncStreamsOfN             1  thrpt         5    94376.318    21107.272    ops/s
r.o.OperatorMergePerf.mergeNAsyncStreamsOfN          1000  thrpt         5        5.372        0.334    ops/s
r.o.OperatorMergePerf.mergeNSyncStreamsOf1              1  thrpt         5  4997823.354   192986.950    ops/s
r.o.OperatorMergePerf.mergeNSyncStreamsOf1            100  thrpt         5   373942.865     9122.578    ops/s
r.o.OperatorMergePerf.mergeNSyncStreamsOf1           1000  thrpt         5    38595.764     4363.691    ops/s
r.o.OperatorMergePerf.mergeNSyncStreamsOfN              1  thrpt         5  5680271.508  1101822.281    ops/s
r.o.OperatorMergePerf.mergeNSyncStreamsOfN           1000  thrpt         5       62.239        3.386    ops/s
r.o.OperatorMergePerf.mergeTwoAsyncStreamsOfN           1  thrpt         5    77968.997     9371.666    ops/s
r.o.OperatorMergePerf.mergeTwoAsyncStreamsOfN        1000  thrpt         5     3609.566      292.903    ops/s
r.o.OperatorMergePerf.oneStreamOfNthatMergesIn1         1  thrpt         5  5330764.814   153222.533    ops/s
r.o.OperatorMergePerf.oneStreamOfNthatMergesIn1      1000  thrpt         5    30944.443     2774.704    ops/s
r.o.OperatorMergePerf.oneStreamOfNthatMergesIn1   1000000  thrpt         5       31.520        0.679    ops/s
```