* nix/libstore/worker-protocol.hh (PROTOCOL_VERSION): Bump to 0x10f.
* nix/libstore/remote-store.cc (RemoteStore::buildPaths): Send the
BUILDMODE when the daemon supports it. Reject invalid values of
BUILDMODE for old daemons.
* nix/nix-daemon/nix-daemon.cc (performOp) <wopBuildPaths>: Read the
build mode when the client supports it.