Fix pprof handlers to not use a db transaction

[dnephin]

Our /debug/pprof handlers are not very useful right now because the middleware around them start a database transaction. We are currently limited to a single DB transaction at a time, so the profile running prevents any other requests from being handled.

In the short term we can address that by:

1. making sure that the authn, authz, "update user lastSeenAt", and "update accessKey last used" operations use a separate transaction (or possibly no transaction). The transaction used by any middleware must be closed before they call the API handler
2. API handlers that do require a transaction should start their own.

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[dnephin]

Still relevant

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[dnephin]

Fixed by #3299, pprof handler now has a separate read-only txn.