Possible memory leak with Eventuals

[gusinacio]

Eventuals are great but they are a simpler and less used version of RxJs.

It's been known by a long time that indexer-service (ts version) and indexer-agent have a memory leak that grows constantly no matter how many subgraphs are being served.

The only thing that runs constantly on a time based and is shared between those components are eventuals.

I propose updating the implementation of Eventuals to use RxJs because it's widely used and well tested.