[FrameworkBundle] Add configuration to allow control over session start on demand.

1. Gives user control over session start on demand mode.
2. Re-introduce flag to allow session listener to manually start session.
[FrameworkBundle] Use more sophisticated validation and configuration.
[HttpFoundation][FrameworkBundle] Keep save auto_start behaviour as in 2.2 and make component values consistent with FrameworkBundle's configuration options.
Fix tests
merged branch drak/start_on_demand (PR #7576)

This PR was merged into the master branch.

Discussion
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[2.3][Session] Give greater control over how and when session starts

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | na
| License       | MIT
| Doc PR        | symfony/symfony-docs#2475

Refs #6036

Gives control over how start on demand works: allowing to turn it on or off and to allow bag access when session is off.

Commits
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f431cb0 Fix tests
1f521d8 Coding standards
2583c26 [HttpFoundation][FrameworkBundle] Keep save auto_start behaviour as in 2.2 and make component values consistent with FrameworkBundle's configuration options.
ceaf69b [FrameworkBundle] Use more sophisticated validation and configuration.
af0a140 [FrameworkBundle] Add configuration to allow control over session start on demand.
8fc2397 [HttpFoundation] Give control over how session start on demand.