mfa: user server instead of log context.Context for audit events
mfa: user server instead of log context.Context for audit events

In `GRPCServer` handlers, `g.Context` resolves to the context included
in `logrus.Entry` due to embedding.
This context is typically `nil`, so if anyone tries using it (such as
the `aws-sdk-go` when using a dynamodb audit backend), things break.

Use the `closeCtx` from the parent `auth.Server` instead.

gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport