Extend symfony DataCollector, do not serialize created_at as \DateTime

[toooni]

Errors with deserialization for the profiling of blocks in debug mode with PHP 7.3

Instead of implementing an own solution for serialize/unserialize which resolves the PHP 7.3 bug https://bugs.php.net/bug.php?id=77302, I modified the BlockDataCollector to extend Symfony\Component\HttpKernel\DataCollector\DateCollector. This way we take advantage of the already solved serialize/unserialize issue there.

I am targeting this branch, because this affects all users with PHP 7.3. This fix keeps BC even for already serialized data.

Closes #556

Changelog

### Changed
- Fix serializing issue for `BlockDataCollector`

[phansys]

Is there a way to add tests for this fix in order to avoid regressions?

[core23]

Ping @toooni

[toooni]

@core23 I've added an object in a test trace which should make a problem if we wouldn't use the AbstractDataCollector and PHP7.3. I don't think this doesn't help to prevent anything since - but I wouldn't know what else to do ;)

[phansys]

@toooni, could you please explain how to reproduce the issue this PR is fixing?
I'll try to check it locally.

[core23]

Ping @toooni

[SonataCI]

Could you please rebase your PR and fix merge conflicts?

[toooni]

@phansys Here's a link to the issue (which is not associated with me) #556
Honestly - I can't reproduce it now. I don't know why and don't have the time right now to dig into it from the beginning (I've reproduced this in March). But relying on the Abstract DataCollector from Symfony reduces complexity (-39 lines of code) in this project and is still a good idea to avoid such issues in the future.

[phansys]

I agree @toooni, thank you.
Could you please squash your commits?

[toooni]

@phansys Can't you just choose "Squash and merge" in the merge drop down menu? Else I can do this next week.

[greg0ire]

Thanks @toooni