Save app changes in bursts

A save now publishes everything since the last save in one commit: the
documents messages edited and the states that changed. If it fails, every
event since the last save is undone (values, text and list drafts, the
list undo log, event and tick counts), so an event reported as successful
is stored and a failed one leaves no trace.

By default each key, text and message event saves before it returns, as
before. With pure_app_defer events apply at once and a host saves a burst
with one commit; a key's document joins a burst once, so revisions move
one per save. On the Pi's SD card 600 events take 6.3 s saved one by one,
0.97 s in bursts of 8 and 0.34 s in bursts of 64. pure-run takes --batch N.

Tests compare a deferred stored app that saves at random and sometimes
stops unsaved with an app that replays only the saved events, undo a whole
burst on a failed save, and kill a process while it saves a burst.

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