Switch away from passing funs between nodes when uploading attachments

[nickva]

Passing closures around is fragile and prevents smooth upgrading. Instead pass
a tuple with a data from the receiver closure explicitly and convert to back to
a local fun locally on each node.

Config options [fabric] attachment_receiver_tuple controls how receivers are
encoded. To upgrade a cluster to use the new format, first upgrade all nodes to
the new code, then flip the value to true. In the future, the option and the
old closure handling code will be removed.

[wohali]

I can't review the code here, but I insist you add something to the documentation about this new flag, even if it's just in rel/overlay/etc/default.ini.

Speaking of which, I don't see any changes in this PR to that file 😞 ALL new config:get entries should be paired with a change to default.ini.

When we change the default value and remove the config option, we will need a big screaming warning in the upgrade docs that says rolling upgrades from older CouchDBs that have the value set to false will fail. (It might be worth describing exactly how those upgrades will fail, too.)

[nickva]

@wohali sorry, this is still a WIP, forgot to mark it as such