Make server act as "aggregator"

[conradoplg]

Closes #251

Basically:

• This moves the SessionState from the Server to the Coordinator, with some modifications
• This removes most functions from the state, adding a new send and receive instead

I ended up doing some refactorings to make the implementation simpler. I should've split that into another PR, sorry!

• I added a ProcessedArgs struct to the Coordinator which is created from a Args, and does all the handling related to reading inputs files or stdin, etc.
• I added support for specifying multiple messages and randomizers in the arguments, but this is still not used by the rest of the code. Tracked in Server cleanups #266

[natalieesk]

Just a little typo, otherwise all great

Change was made