Handling refresh tokens

[naicigam]

Based on #241, I am doing a wrapper to handle token expirations. For each operation I am doing something like:

try
{
    return await _client.CreateAsync(record.SObjectType, record);
}
catch (ForceException e)
{
    await RefreshTokenOrRethrowException(e);
    return await _client.CreateAsync(record.SObjectType, record);
}

which is pretty verbose.

So my first question is: should we handle token expiration inside the ForceClient? I could work on a PR for this.

My next question is regarding bulk operations: for example RunJobAndPollAsync makes several API calls and each of these should handle a potential token expiration. If we handle them in the ForceClient class, we could do it in the individual functions, like RunJobAsync and PollBatchAsync, but if not, what would be a good strategy to deal with this?

Thanks!