Make global `$all` stream building a separate task

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All events appended to a stream are linked to the globally ordered all events stream (named $all). Because the events must be assigned a unique, monotonically incrementing, and gapless event number the linking is blocking, resulting in single writer performance.

To improve append event performance the all stream building could be moved to a separate async process which won't block event inserts into a stream. It should also be possible to disable all stream building via a config option.

Running the existing append events benchmark with/without all stream building resulted in a significant speed up.

AppendEventsBench

• append events, 10 concurrent writers -79.53%
• append events, 50 concurrent writers -71.69%
• append events, 20 concurrent writers -59.88%
• append events, single writer -38.53%

Above comparison shows the difference in time between all stream building enabled and disabled (commented out).

[slashdotdash]

This feature could be combined with the changes needed for #169.