Contrary to the existing Task Sink, the Async and Sync Mutex Sinks
handle buffers in the `PadSinkHandler` directly. The Async Mutex
Sink uses an async Mutex for the `PadSinkHandlerInner` while the
Sync Mutex Sink uses... a sync Mutex.
All Sinks share the same settings and stats manager.
Use the `--sink` command line option to select the sink (default is
`sync-mutex` since it allows evaluating the framework with as little
overhead as possible.
Also apply various fixes:
- Only keep the segment start instead of the full `Segment`. This
helps with cache locality (`Segment` is a plain struct with many
fields) and avoids downcasting the generic `Segment` upon each
buffer handling.
- Box the `Stat`s. This should improve cache locality a bit.
- Fix EOS handling which took ages for no benefits in this
particular use case.
- Use a macro to raise log level in the main element.
- Move error handling during item processing in `handle_loop_error`.
This function was precisely designed for this and it should reduce
the `handle_item`'s Future size.
