Redresses a previous holdover from an attempt to get Restartable sources to work more neatly inside the synchronous mixer thread. This prevents `Restartable::*` from blocking without warning.
The initial fix at the time was to perform the restart work on a task provided by the tokio runtime as `executor::block_on` needs to be run from within a valid async runtime. Naturally, this completely missed the point that these closures should/could be async, without any need to fudge async functions into a sync wrapper.
Also removes the `From` for normal closures, as this will probably act as a footgun for folks on a single-threaded executor.
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