wfe: propagate cancellations

[jsha]

In #5459 and #6608 we prevented propagation of cancellation from clients to SA / WFE. This was an attempt to solve a performance issue we found when upgrading go-sql-driver/mysql: #5437. That was speculative, however. We never confirmed that cancellations were causing connection churn. And we now have a better theory as to what caused the problem: #7556.

Now we have been having some performance problems that appear to be related to requests piling up and new requests timing out before any work gets done. This is the sort of situation that cancellation helps to solve.

We should (behind a feature flag) re-enable propagation of cancellations.