daemon: Client settings no longer override daemon settings.

daemon: Client settings no longer override daemon settings.

Fixes <http://bugs.gnu.org/20217>.

* nix/libstore/worker-protocol.hh (PROTOCOL_VERSION): Bump to 0x161.
* nix/nix-daemon/nix-daemon.cc (performOp): "build-max-jobs",
"build-max-silent-time", and "build-cores" are no longer read upfront;
instead, read them from the key/value list at the end.
* nix/nix-daemon/guix-daemon.cc (main): Explicitly set
'settings.maxBuildJobs'.
* guix/store.scm (%protocol-version): Bump to #x161.
(set-build-options): #:max-build-jobs, #:max-silent-time, and
 #:build-cores now default to #f.  Adjust handshake to new protocol.
* tests/store.scm ("build-cores"): New test.
* tests/guix-daemon.sh: Add test for default "build-cores" value.