Nonblocking rewards sink for tests

[michaeldjeffrey]

Provide a testing FileSink that acks messages as they're written so we don't have to deal with race conditions in tests.

Previously

    let (_, rewards) = tokio::join!(
        rewarder::reward_mappers(&pool, &mobile_rewards_client, &reward_info),
        receive_expected_rewards(&mut mobile_rewards)
    );

Tests required that you know how many messages you were expecting, otherwise the test would hang indefinitely either waiting for a written message to be acknowledged, or waiting for a message that would never be written.

Now

The test FileSinkReceiver consumes all messages that are written as they come.
Functions in the rewarder now own a FileSinkClient<T>

pub async fn reward_poc_and_dc(
    pool: &Pool<Postgres>,
    hex_service_client: &impl HexBoostingInfoResolver<Error = ClientError>,
    mobile_rewards: FileSinkClient<proto::MobileRewardShare>,
    speedtest_avg_sink: &FileSinkClient<proto::SpeedtestAvg>,
    reward_info: &EpochRewardInfo,
    price_info: PriceInfo,
)

Cloning a FileSinkClient<T> is relatively cheap as it's just a channel and a metrics string.
When the rewarding function under test is done, it will drop the Sender, and we can know that all messages that will be written have been written.

We can then make assertions against all the messages written during the test. Whether they were expected or not.