Run daemon on a server or in a different container

[mverleg]

It should not be very hard to make the daemon run on a server and have the CLI connect to it remotely.

This may be useful in a compiler-farm setup, or in a CI pipeline where a build-container keeps a continuous cache.

Some things that'd need to be addressed:

• Lockfiles and stopping daemons would be more limited, probably without ability to kill unresponsive daemons.
• Source file content needs to actually be sent to the daemon, with a clever way to send only unknown files.
• Add a command to connect to a remote daemon, which'd be used instead of start. Also similar flags for build etc.
• Should the run subcommand run locally or remotely? (Probably local)

For now let's not include having multiple daemons with a load balancer, though it might be possible one day.

[mverleg]

It'd also be useful for sharing a daemon between container and host OS.

But it may be a bit more tricky than I had thought. Some ideas:

• Remote dependencies can be sent or downloaded, but either way they shouldn't change much
• Whichever way is used to send source files do daemon: does it also work for non-source material? Ideally only compile-time things, like what is included by macros. Sending everything would be a big slowdown.
• Should intermediary artifacts be sent back, or just final executable/lib?

The problem with non-code resources is actually not just the daemon - the compiler should also know about such things, to be able to rebuild when needed.

EDIT: The problem in general, and especially with non-source, is that one either 1) sends everything in a batch up-front, or 2) sends only what is needed, which means lots of back and forth communication, because it only becomes known gradually.