Avoid special-case handling in ServiceEntryPointImpl::shutdown

Shutting down the service entry point outside TSAN and ASAN builds immediately returns true without running any shutdown code. A separate interface, shutdownAndWait, is introduced to shutdown the service entry point.

bool ServiceEntryPointImpl::shutdown(Milliseconds timeout) {

#if __has_feature(address_sanitizer) || __has_feature(thread_sanitizer)

    // When running under address sanitizer, we get false positive leaks due to disorder around

    // the lifecycle of a connection and request. When we are running under ASAN, we try a lot

    // harder to dry up the server from active connections before going on to really shut down.

    return shutdownAndWait(timeout);

#else

    return true;

#endif

}

We should remove the special-case handling in shutdown and have it run the body of shutdownAndWait. This would also obviate the need for shutdownAndWait.

Acceptance criteria: Change the code to have #if at the higher level of the stack. Make sure we link these to the relevant shutdown project.