Vine: Forsake functions when library exits.

[dthain]

Proposed changes

Previously, when a library would exit (success or otherwise), two problems resulted:
1 - The function calls sent subsequent to that library would simply sit on the worker doing nothing.
2 - The manager would quickly re-dispatch a library to that same worker, resulting in a tight loop of failures.

This PR fixes these as follows:
1 - The worker now has extended logic for task_can_fit_eventually so that functions will be forsaken if there is no library process at all that could execute them.
2 - Forsaken tasks are now handled correctly by deleting their sandboxes once they are retrieved.
3 - The most recent failure of a library is recorded in the library prototype object and used to throttle re-dispatch of failed library tasks.
4 - Fetching of task stdout is factored out of get_result to make the logic of fetching a result more clearly visible.

Post-change actions

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Additional comments

This section is dedicated to changes that are ambitious or complex and require substantial discussions. Feel free to start the ball rolling.

[dthain]

Still thinking about this one in two respects:
1 - The FORSAKEN code path in the manager is not overly trod, I want to make sure this behaves as expected.
2 - This will still cause the manger to retry libraries quickly, we need some kind of hysteresis in library attempts.

[dthain]

Since this is an improvement to a corner case and not a critical bug, I think it can wait until after the pending release...