unify send_request and send_command

[shashi]

I want to use evaljs in a fire and forget kind of way. I'm trying to send 1000s of these in a for loop, it works blazing fast if I just remove the code that sets up the future and the waiting tasks but of course prints a ton of "Warning: response received for unknonwn....".

Ideally, send_command would just be something like send_request(blah...; async=true) and evaljs can take async as an argument which it forwards to send_request.

Cheers

[twavv]

Closed in favor of #371/#372.

tl;dr:

• command is fire-and-forget
• request sets up the tasks and futures
• Individual functions can choose the appropriate message type.
• evaljs has can be called with result=true to use a request, but the default is to use a command (especially since this is the "safer" option)