Perf[BMQIO]: remove unnecessary function copy

[678098]

IO thread should spend 7.5% (1.7 / 22.96) less cpu

Before:

After:

[678098]

    bslmt::UnLockGuard<bslmt::Mutex> unlock(&d_mutex);
    // !!!!!!!!! `close` might be called from another thread
    readCallback(bmqio::Status(), &numNeeded, &d_readCache);

Here, we have a non-trivial thread race. In the past, we could have emptied the d_callback of NtcRead between these 2 lines of code. We make a copy of d_callback before unlocking a mutex, so we don't access the empty callback. However, it means that we can send another callback from this copy after we executed close.

My PR doesn't fix this race, so we still can send something over this callback. Instead, I make sure not to clear it from another thread, so this reference is always alive. And I also added a post-check for isComplete() to return early.

[bmq-oss-ci]

Build 341 of commit 699d567 has completed with FAILURE

[dorjesinpo]

Maybe, we can eliminate setComplete now and use clear instead?

[678098]

Maybe, we can eliminate setComplete now and use clear instead?

Done. Made sure that we have clear down the code paths where I removed setComplete.

[bmq-oss-ci]

Build 346 of commit f0fcea3 has completed with FAILURE