Combine forward and backward dependencies

[angelhof]

TODOS:

• Run non-first commands in sandbox (modify executor.py)
• Run all commands asynchronously (modify orch.py)
• Check dependencies and decide whether to commit sandboxes or not (probably modify orch.py)

[angelhof]

@gliargovas the final TODO is described in a comment here (https://github.com/binpash/dynamic-parallelizer/blob/forward_backward_combination/overlay-sandbox/run-sandboxed.sh#L42). We essentially need to do the sandbox-file-change check inside orch, and only commit the write changes in the upperdir of each sandbox if they do not interfere with the changes of the non sandboxed command.