Added config to force Symfony to use native session handler by default

Added comment for handler_id
backported #585
feature #606 Added config to force Symfony use native session handler by default (pulzarraider)

This PR was submitted for the master branch but it was merged into the 2.4 branch instead (closes #606).

Discussion
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Added config to force Symfony use native session handler by default

As far as I know Symfony is using NativeFileSessionHandler to save session data to file. This breaks server configuration, because if php is configured to use Memcache or Redis session handler, symfony stores session data to file in app/cache directory and ignores default php session settings.

Correct me if I am wrong, but Symfony should use native session handler that is configured in php ini settings by default (to create less WTFs).

Commits
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89f82c8 Added config to force Symfony to use native session handler by default
updated to FrameworkExtraBundle 3.0 (examples updated as well)
updated VENDORS for 2.4.0-RC1
bumped 2.4 to stable
customization
Merge branch '2.4'