[V2] Simplify session system, make it more robust and add new features

[LoicPoullain]

Issue

This PR solves many issues:

• Allow to regenerate the session ID for security reasons ([Sessions] Allow to regenerate the session ID #728) .
• Make it easier to implement a custom session store and maintain it ([V2][Sessions] Simplify the implementation of a session stores. #794).
• Cleanup regularly expired sessions in MongoStore and TypeORMStore ([V2][Sessions] Cleanup regularly expired sessions #793).
• Reduce the SQL database size.
• Make the code more concise when using session.
• [Internal] Remove hacks
• Make it easier to use templates and to authenticate anonymous users ([V2][Sessions] Make it easier to use templates and to authenticate anonymous users #795)

Solution and steps

Breaking changes

• Except for the get and set methods, the interface of Session has changed.
• The session timestamps saved in the databases are in seconds, no longer in milliseconds.
• The function setSessionCookie takes the session as argument, no longer the token lonely.
• Implementing a session store is different.
• To read or create a session, use the function createSession and readSession instead of the store directly.
• createSession(..., { csrfToken: true }) option is removed for now. Use session.set(csrfToken, await generateToken()) instead.
• Only numbers for user IDs are allowed in TypeORMStore.
• TypeORMStore.getSessionsOf becomes TypeORMStore.getSessionIDsOf
• getRedisInstance and getMongoInstance removed from the stores.

// Before
const session = await this.store.createAndSaveSessionFromUser(user);

// After
    const session = await createSession(this.store);
    session.setUser(user);
    await session.commit();

TODO at the end: check that the collection and tables names for sessions (redis, mongodb, typeorm) are different from the ones in v1.

Checklist

• Add/update/check docs (code comments and docs/ folder).
• Add/update/check tests.
• Update/check the cli generators.