[HttpFoundation] fix PDO session handler under high concurrency
[HttpFoundation] use MERGE SQL for MS SQL Server session storage
[HttpFoundation] use insert or replace for sqlite session handler
bug #10652 [HttpFoundation] fix PDO session handler under high concurrency (Tobion)

This PR was merged into the 2.3 branch.

Discussion
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[HttpFoundation] fix PDO session handler under high concurrency

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8448 and http://trac.symfony-project.org/ticket/4777 (which was never really fixed as you can see here)
| License       | MIT

- The first commit fixes PDO session handler under high concurrency.
- The second commit uses MERGE SQL for MS SQL Server. Tested with http://sqlfiddle.com/#!6/66b6d/14
- The third commit uses INSERT OR REPLACE for sqlite session handler http://sqlfiddle.com/#!7/e6707/3

What I find rather bad with the class design is that it depends on the table definition, but it's not part of the class. Also it doesn't make use of open() and close() which could be used to make the database connection lazy instead of having is open all the time when not needed. Doctrine also only lazy connects, but we use PDO directly here.
Furthermore, the session handlers should not throw exceptions, from what I read, but return false when an error occurs. This is not followed in this class. Maybe @drak knows how php session management behaves when the session handlers return false?

Commits
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5c08e29 [HttpFoundation] use insert or replace for sqlite session handler
05ea19a [HttpFoundation] use MERGE SQL for MS SQL Server session storage
e58d7cf [HttpFoundation] fix PDO session handler under high concurrency