Resolve a deadlock

[OlhaVorobiova]

Fixes #3817

This PR is ready for review.

Risk

This PR makes no API changes.

Summary

Deadlock is fixed in EventDispatcherImpl class.
Locks are added for map of requests in RequestTimeoutHandlerImpl class. Refactoring in EventDispatcherImpl:

• observers_ is a redundant member of class EventDispatcherImpl, because the function raise_event uses local variable (ObserverVector observers;)
• It is not correct to use erase for mobileobservers, but use removeif for **observers** in remove_mobile_observer_from_vector.

CLA

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[OlhaVorobiova]

@theresalech this PR is ready for Livio review