Avoid passing some function arguments by value

[ryanofsky]

Preemptively avoid clang-tidy errors before clang-tidy is introduced in #83. Other clang errors are already fixed in that PR, but these aren't maybe due to running different clang-tidy versions. The errors look like:

include/mp/proxy-types.h:162:85: warning: the parameter 'perhaps' is copied for each invocation but only used as a const reference; consider making it a const reference [performance-unnecessary-value-param]
                            .then([&server, invoke, req](kj::Maybe<Thread::Server&> perhaps) {
                                                                                    ^
                                                         const                     &
example/printer.cpp:27:39: warning: the parameter 'message' is copied for each invocation but only used as a const reference; consider making it a const reference [performance-unnecessary-value-param]
void LogPrint(bool raise, std::string message)
                                      ^
example/printer.cpp:29:41: warning: passing result of std::move() as a const reference argument; no move will actually happen [performance-move-const-arg]
    if (raise) throw std::runtime_error(std::move(message));
                                        ^~~~~~~~~~       ~