Fix concurrency errors when expanding columns on a frame (#340)

[chrisharding]

A simple fix for #340 as discussed in the issue.

If a different solution would be better, I'd be appreciative if someone could address it quickly.

Thanks.

[tpetricek]

Sorry about the long delay here. There were quite a few changes there were, unfortunately, unmerged, which caused this PR to no longer work.

@adamklein What are your thoughts on this? I think making this thread-safe would be good (although we do not really have a consistent policy on this).

Do you think there is any negative perf hit for the single-threaded scenario? (Perhaps using a .NET concurrent dictionary would be better, if that's available in .NET 4.0 that we're currently targetting...)

[tpetricek]

Ping @adamklein - do you have any opinion on this?

I think making this thread safe would be good, but I'd go with concurrent dictionary instead.

[dsyme]

See #394 instead