This PR was merged into the 2.6-dev branch.

Discussion
----------

[HttpFoundation] enhance PdoSessionHandler

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #5483, #2067, #2382, #9029
| License       | MIT

0. [x] Continuation of locking implementation (#10908): Implement different locking strategies
  - `PdoSessionHandler::LOCK_TRANSACTIONAL` (default): Issues a real row lock but requires a transaction
  - `PdoSessionHandler::LOCK_ADVISORY`: app-level lock, safe as long as only the PdoSessionHandler accesses sessions, advantage is it does not require a transaction (not implemented for oracle or sqlsrv yet)
  - `PdoSessionHandler::LOCK_NONE`: basically what is was before, prone to race conditions, means the last session write wins

1. [x] Save session data as binary: Encoding session data was definitely the wrong solution. Session data is binary text (esp. when using other session.serialize_handler) that must stay as-is and thus must also be safed in a binary column. Base64 encoding session data just decreses performance and increases storage costs and is semantically wrong because it does not have a character encoding.
That saving null bytes in Posgres won't work on a character column is also documented

    > First, binary strings specifically allow storing octets of value zero and other "non-printable" octets (usually, octets outside the range 32 to 126). Character strings disallow zero octets, and also disallow any other octet values and sequences of octet values that are invalid according to the database's selected character set encoding.
http://www.postgresql.org/docs/9.1/static/datatype-binary.html#DATATYPE-BINARY-TABLE

2. [x] Implement lazy connections that are only opened when session is used by either passing a dsn string explicitly or falling back to session.save_path ini setting. Fixes #9029

3. [x] add a create table method that creates the correct table depending on database vendor. This makes the class self-documenting and standalone useable.

5. [x] add lifetime column to session table which allows to have different lifetimes for each session

6. [x] add isSessionExpired() method to be able to distinguish between a new session and one that expired due to inactivity, e.g. to display flash message to user

7. [x] added upgrade and changelog notes

Commits
-------

1bc6680 [HttpFoundation] implement different locking strategies for sessions
6f5748e adjust sqlite table definition
5978fcf added upgrade and changelog notes for PdoSessionHandler
182a5d3 [HttpFoundation] add create table method to pdo session handler
e79229d [HttpFoundation] allow different lifetime per session
af1bb1f add test for null byte in session data
251238d [HttpFoundation] implement lazy connect for pdo session handler
7dad54c [HttpFoundation] remove base64 encoding of session data