Use random temporary file name for rserver secure cookie key

[julozi]

When using the batchspawner, we can have several rserver session started on the same cluster node. However, rserver is by default using the same path /tmp/rstudio-server/secure-cookie-key to store secure cookie key. As a result the cookie key file is locked and owned by the first user starting a RStudio session on a given node.
To avoid this, we need to define a distinct temporary file for rserver secure cookie key for each rserver instance.

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[ryanlovett]

This is great, thanks very much! One request: could you please omit prefix='/tmp/' ? That way we could let tempfile determine the directory. Or is /tmp a hard requirement for rstudio?

relevant #64

[danielfrg]

I can confirm that this totally works on master and you don't need the prefix='/tmp'.

@julozi would you be able to update this branch to master? If not I can do it.

[ryanlovett]

Fixed by #98.