refactor(ethexe): Subscribe blocks when Observer is created

[techraed]

Resolves #4099

TODO:

• use macro to avoid copy-paste in EventsStreamProducer

UPD:
Still partially resolves #4099

[techraed]

I'd say that tests are really fragile: removing timers results in underlying spawned tasks being not synchronized, which creates many obstacles in testing. The approach to test impl should be changed in future, so we can write them easily without thinking too much about underlying details of how Service and other entities are implemented.

6e450c4 - this works perfectly fine, but requires some timeout for sync.

a465d72 - this avoids sync problems when channgels try to read data, when there's no actual data sent from the anvil, because it works in a non-block-production mode. However, it requires increasing a number of attempts to get the receipt for the sent tx (could be the anvil problem).

[techraed]

After some discussions we came to the conclusion that having blocks automatically produced is not a good solution, so dividing the PR to the 2:

• subscribing to blocks during Observer instantiation
• actually solving sleep problem