Change `BatchingQueueCallInvoker` to re-use task queue for each batch

[jonthysell]

Summary

Right now, BatchingQueueCallInvoker creates a std::vector with initial capacity of 2048 elements to store the batch of tasks that will eventually be std::moved to the UI thread to be processed.

Instead of allocating and moving the vector each time, instead find a way of reusing the vector.

Motivation

Some internal partners have automated testing to catch memory leaks using memory-constrained environments, which catch OOM errors during memory allocations. Despite not being a leak, and its relatively small size, the constant (re-)allocation of this vector means it often triggers the OOM and gets blamed for others' leaks.

Finding a way to prevent the constant allocation would allow any actual leaks to be correctly root caused and reduce the noise blaming RNW.

Basic Example

No response

Open Questions

Since the tasks need to be moved onto the UI thread, is it even possible to do this?

[vmoroz]

In Office DispatchQueues we have two vectors - one for reading and another for writing - which we swap as soon as we finish handling all items in the reader vector. Thus, after some time we almost never allocate the memory again. (It also allows us to avoid locks while reading items from the reader vector.)
We can implement similar approach here.
I can implement the fix.