Fix the allocator for Flow Graph critical tasks creating

[kboyarinov]

Description

There is a bug in the current implementation of Flow Graph while using functional node concurrency limits together with node priorities.
Current algorithm for creating tasks in function node is the following:

• If the concurrency limit is not reached, two tasks are created - graph_task for executing the body and priority_task_selector task for prioritization. Both tasks are created by the same thread.
• If the concurrency limit is reached, the postponed items are stored in the queue.
• Once the resources of the node are available, the thread X tries to get the postponed item from the queue using the aggregator.
• Since the aggregator is used, another thread Y can take the "aggregated" work and hence, create the graph_task using its small object pool.
• Once the control goes back to thread X, it creates a priority_task_selector wrapper. The bug in the current implementation is that the small object pool, associated with the underlying task (created by thread Y) is used instead of small object pool of thread X.

Also this patch adds extra assert for small object pool to ensure that the pool, for which the allocation is requested is the same as the TLS pool.

Fixes #1595

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• added - required for new features and some bug fixes
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[aleksei-fedotov]

Looks good to me!