fix: provide remote servers a way to find out about an event created during the remote join handshake

[FrenchGithubUser]

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TLDR

Use a "dummy" event to tie together forward extremities, and proactively send it to all servers in the room. This allows recently joined servers to become aware of recent events that would otherwise have "slipped through the cracks" and thus not be retrievable.

NOTE: While this does send the "dummy" event to all servers in the room, regardless of if they should care or not, at some point a new event will reference this dummy event and require it's retrieval. Since it was proactively sent, this will now not be necessary. This assists in preventing forks in the DAG

Alternatives

Unlike famedly/synapse#51 which 'pushes' the missing event directly, this causes the event to be 'pulled' by referencing it as a prev_event of a dummy event. Since the 'dummy event' does not get passed into the client, it is effectively invisible.

Draw-backs of famedly/synapse#51 meant it was not always certain if the 'pushed event' would show up in /sync or in /messages, but usually was in /sync. This method always has the 'missing event' show up in /messages, which I feel is more technically correct as that event was(albeit just barely) created before the 'join event' is persisted.

The Process

The order of events:

1. make_join from remote server, response sent
2. Message A sent from local server
3. send_join from remote server, response from local server. Message A is not in this(as it is not state and is not referenced in any events that are included). Join event is persisted on local server.
4. Local server realizes there are two forward extremities just after persisting the join event.
   A. Creates a org.matrix.dummy_event that has prev_events containing both the join and message A.
   B. Sends this dummy event to all servers in the room.
5. Remote server receives the dummy event via it's /send endpoint, saves it in a queue until the partial state join begins syncing additional room state

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[FrenchGithubUser]

I am submitting this PR as an employee of Famedly, who has signed the corporate CLA, and used my company email in the commit.