Improve perf of token cleanup

[feanz]

resolves #89

Use entity framework extended to run the token clean up process. This pevent the cleanup code pulling the data back before deleting it.

Add an index to the expiry field to improve performance of selection.

This current solution does not include @tbarcelon proposed plan to move the clustered index to expiry. It just adds an extra non clustered index.

Note: I've included a test that I would remove before merging this code it just so you can run it quickly run it to check it works.

I've profiled the sql that gets executed by entityframework extended.

DELETE [dbo].[Tokens]
FROM [dbo].[Tokens] AS j0 INNER JOIN (
SELECT 
    1 AS [C1], 
    [Extent1].[Key] AS [Key], 
    [Extent1].[TokenType] AS [TokenType]
    FROM [dbo].[Tokens] AS [Extent1]
    WHERE [Extent1].[Expiry] < '2016-05-13 14:19:03.6501655 +00:00'
) AS j1 ON (j0.[Key] = j1.[Key] AND j0.[TokenType] = j1.[TokenType])

Included execution plan

Less than ideal compared to the alternative but probably good enough.

DELETE FROM [dbo].[Tokens] WHERE [Expiry] < '2016-05-13 14:19:03.6501655 +00:00'

[dnfclas]

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[dnfclas]

@feanz, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.
Thanks, DNFBOT;

[brockallen]

Just got around to this now -- I like the idea and sorry for the delay.