Add support for builtins.filterSource, refine builtins.readDir

The filter of filterSource is only respected for direct children of the source root. For example, if you use nix-gitignore, .git is not watched.

builtins.readDir’ed paths are now not watched recursively, which should greatly reduce inotiy resource consumption.

Fix watcher not starting if the first nix build fails.

We were not actually watching the nix file, only indirectly picking it up from the first nix build; so if that failed, we’d not watch any file.

Now lorri will always add the shell.nix to the watchlist, ensuring it triggers a rebuild as soon as it is fixed again.

Various fixes to lorri internal stream-events after daemon socket protocol rewrite. It would hang and leak file descriptors in the daemon.