Reduce mutex contention in ActiveStream::next during DCP backfill

As seen during investigation of MB-58013, there is significant mutex contention on ActiveStream::streamMutex during ActiveStream::next (on frontend worker threads), and ActiveStream::backfillReceived on AuxIO threads:

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Note that ~12% of worker thread time and ~13% of AuxIO thread time is spent locking and unlocking these mutexes.

I don't believe the issue here is the lock duration / scope, it's more there is contention given around 10% of time in each thread is spend in the kernel calling futex - i.e. userspace finds a locked mutex and hence must call into the kernel to acquire it once unlocked by the other thread.

We therefore probably want to look into reducing the contention - perhaps avoiding taking the "big lock" streamMutex in this path and communicate via some form of concurrent queue...