vine: consider cached output buffers as replicas at workers

[btovar]

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Additional comments

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[btovar]

vine_file_has_changed uses st_size. This is an issue for directories because we would like to have f->size to be the total size of the files inside the directory for scheduling purposes, but we don't want to stat every time all its contents every time to check if it has changed.

One easy fix is to check if the payload is a directory, and if so, drop the check against st_mtime.

@dthain