Move check for reader to action function for message store GC.

Message store GC postpones processing of file, which have readers.
When performing an action, it asserts that there are no readers.

Check for readers may race with readers update by a queue, crashing
the message store.

Make check and assert work with the same lookup to reduce failure rate.

In case of races the queue process should handle exception instead.

Addresses #2000
[#165755203]

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