Need better cleanup when aborting a build / test run

[jasonmccampbell]

Biggest issue: server leaves .spraycc-history file laying around when cancelled. If done in a subdirectory this can cause a run from the top-level to fail when clients start in the subdirectory and attempt to connect to the wrong server.

Second issue: ctrl-C of make terminates the clients, which disconnect from the server, but don't stop any of the jobs. If the server has 1k jobs, it will continue feeding the execs for no reason. If the server is terminated, see above.

Third issue: even if the server is terminated, the executors continue running the tasks to completion before exiting. This may or may not be a long time. This is because tasks are run synchronously instead of asynchronous so the disconnect goes unnoticed.

In general, should ctrl-c of one client cause that one task to be removed? Is it work the bookkeeping to do that? Or is the general case all clients are terminated, at which point the server can go into shutdown mode?