Small socket server

[RagnarGrootKoerkamp]

Inspired by #41, but more minimalistic, this add a small deacon server mode that opens a unix socket in its working directory.

Then running more commands like deacond --use-server filter ... will forward all arguments to the server, which will process them.

Does not support networking, nor stdin/stdout (TODO add some checks for this?), since all IO is done by the server.

As said before, main benefit here is that you don't have to wait for the index to load on each invocation.

Limitations:

• you'll have to clean up the deacon_server_socket file manually between runs
• for now each server only supports a single index. Should be fine anyway.

Maybe we should restrict the commands sent to the server to only Filter? All index construction/modification commands aren't cached anyway.

TODO: pass the -t THREADS flag to the server invocation and use that instead of whatever the first client passed.

[bede]

Merged, need to add some tests for this later

[RagnarGrootKoerkamp]

Ah there's also still a TODO in the first post here that would be nice to fix.

(Fixed in #50)