add `.expr=` arg to `launch_*()`

[r2evans]

Stemming from #539, if we add one or more connections, it would be good to be able to run a starting expression on them before they are passed new tasks.

Consider this:

daemons(4, idletime=300)
everywhere(library(dplyr))
# oops, I really need more connections
launch_local(4)

In the current state, some have dplyr loaded and some do not. In this case it is trivial to call everywhere(library(dplyr)) again, but it's not hard to imagine either due to time, bandwidth/serialization, or some other reason that we would not want to re-everything() to all connections, just to the new connections.

Options:

• launch_*(.expr=) so that the new connections are established and the .expression is executed immediately
• add the ability to subset the connections somehow, which requires both (a) ability to request the "id" (or whatever) of each connection, and (b) pass the list of ids to everywhere()

As a hack, if I were using ssh_config(), perhaps I might play around with rscript= so that it loads R with an initial expression. Not sure if that would actually work, but ... I'm not currently using ssh_config().

[shikokuchuo]

Possibly, but likely only if it supports some particular functionality that depends on it. @wlandau requested some setup argument to new launches previously, but I think he found a work-around.

I'd expect you to handle where everywhere() has already run by e.g. querying the existence of a sentinel value, which should be quick.

[r2evans]

Are you referring to #490?

[shikokuchuo]

Yes, so now we have 2 placeholders for this FR.