Use REQUEST_TIME_FLOAT if available.

This will give a more correct initialization time when using the
DataCollectors without a KernelInterface implementation such as Silex.
merged branch henrikbjorn/time-collector (PR #6971)

This PR was merged into the master branch.

Commits
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bd0709c Use REQUEST_TIME_FLOAT if available.

Discussion
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Use REQUEST_TIME_FLOAT if available.

This will give a more correct initialization time when using the
DataCollectors without a KernelInterface implementation such as Silex.

`REQUEST_TIME_FLOAT` is available as of 5.4.0

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by henrikbjorn at 2013-02-05T09:59:05Z

@fabpot Should be good now with passing test :)