Parallel ast.Walk has errors with BinaryExpr

[jacksontj]

I was digging some more into #451 and it seems that there is some issue with parallel execution of ast.Walk specifically for BinaryExprs. Unfortunately I have no good lead at this point on what that may be but that was where I seemed to find an issue. Creating this to track a longer-term fix. For now I'll revert the parallel execution in the latest prom fork which I'm working to upgrade to (#460)

[jacksontj]

A note to myself; I found a few places in the promql engine that use the Inspect and path assuming a specific traversal order; so we'll have to modify the prom fork to handle the parallel execution.

[jacksontj]

WIP in jacksontj/prometheus#2

At this point I've fixed the data races; but still have one ordering issue in a test case.