avoid revision conflict during deletion
purge empty fdocs on select
move mbox-doc handling to soledadstore, and lock it
remove all refs during removal, and protect from empty docs
select instead of examine
docstring fixes
suggest bigger threadpool to reactors that honor it
remove early notification on append for now

this can be done to save some msec, but additional measures
have to be taken to avoid inconsistencies with reads right after
this is done. we could make those wait until a second deferred
is done, for example.