Update EventIndex crawler to discover threads timelines

[germain-gg]

matrix-react-sdk/src/indexing/EventIndex.ts is the abstraction built over seshat to crawl through events in e2ee rooms.

It leverages the js-sdk events to discover rooms and timelines and will then individually process events and add them to the message index.

We need to rework to EventIndex to make it discover the threads timelines so that it can index the messages

Once all the messages have been indexed, we should go ahead and disable threads labs flag and ensure that we don't need to crawl through the events again. We shouldn't need to as seshat is unaware of threads and it shouldn't ever be aware of them

Requirements

• After login into Element having threads enabled, seshat should parse and index all events (threaded and non-threaded)
• After disabling threads, previously indexed events that are part of a thread should still show up in the search results
• Indexed events should have a way to identify them as part of a thread so that the UI layer can render them differently