emqx / mria / 1183

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feat: let the caller decide whether a write runs in a middleman process

Since b857b2a every write operation runs in a helper process when the
caller's message queue has `max_mql' (default 10) or more messages. The
helper protects mnesia transactions: their commit stage does receives
without the ref trick, so each receive scans the whole queue. A dirty
operation on a table with a local copy never waits for a message, so for
it the helper is pure cost: a spawn, a monitor, and a wait for the helper
to be scheduled. On a loaded node that wait is a run-queue round trip
per write.

This adds the `middleman' option, `#{middleman => boolean()}', as a new
last argument of transaction/4, async_dirty/4, sync_dirty/4,
dirty_write/3, dirty_delete/3, dirty_delete_object/3 and
dirty_update_counter/4. `false' runs the operation in the caller, `true'
always uses the helper, and an absent option keeps the queue-length
check. The existing arities are unchanged.

EMQX will pass `middleman => false' for the route writes that the broker
pool workers do on the v3 routing schema, where the tables are local
content merge tables. Under a subscribe load of about 10,000 new routes
per second per node, one route write in ten already took the helper
detour with the default threshold.

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