core implementation and tests from https://gitlab.com/famedly/conduit/-/merge_requests/467


feature flag, base64 encode update, and tweaks were me

Signed-off-by: strawberry <strawberry@pupbrain.dev>

This speeds up handling of /_matrix/federation/v1/send/:transaction_id
when more than one event contains unknown keys.

In particular, when receiving multiple PDUs with dead servers in their
auth chain, timeouts of each server accumulate and can make handling of
incoming requests take several minutes, to the point the client closes
the connection (eg. matrix.org has a 2 minute timeout), causing new
events to be dropped eventually.

Signed-off-by: girlbossceo <june@girlboss.ceo>

Signed-off-by: girlbossceo <june@girlboss.ceo>

https://rust-lang.github.io/rust-clippy/master/index.html#op_ref

https://rust-lang.github.io/rust-clippy/master/index.html#str_to_string

https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes

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