Motivation for this change

In the current implementation, there's no possibility to modify the default
parameter for keepalive. This is a number that indicates how frequently
keepalive messages should be sent from the worker to the buildmaster,
expressed in seconds. The default (600) causes a message to be sent to
the buildmaster at least once every 10 minutes.

If the worker is behind a NAT box or stateful firewall, these messages
may help to keep the connection alive: some NAT boxes tend to forget about
a connection if it has not been used in a while. When this happens, the
buildmaster will think that the worker has disappeared, and builds will
time out. Meanwhile the worker will not realize than anything is wrong.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.