Feature/10 clean session store files

[ppxl]

solves #10 and #11

This PR introduces a background cleaning task for files created by the FileSessionStore.

• The task iterates all JWTs, and filters for the expiration date. Then it proceeds to remove the JWTs and the associated service ticket file
• The task run interval is configurable with the property sonar.cas.sessionStore.cleanUpIntervalInSeconds.
• Especially for the file removal great lengths were taken (unit test-wise) to ensure only the right files are removed.

This PR adds additionally more unit and integration tests, and elaborates the documentation quite a bit

As usual, larger code parts were extracted and isolated for better maintainability and readability. Most specifically:

• CasIdentityProvider functionality is split into LoginHandler and LogoutHandler
• TicketValidation parts extract into a TicketValidationFactory