Refactoring proposals for session related code

[surbhiia]

Proposals

• Move SessionManagerImpl & SessionIdTimeoutHandler inside a package called "internal" under :session module. It allows others to use the :session module as is, if they wanted to use our opinionated implementation of the SessionManager (i.e SessionManagerImpl) too.

• Similarly, SessionIdLogRecordAppender, SessionIdSpanAppender & SessionIdRatioBasedSampler can be moved in the same module under a "processors" package.

• Rename the SessionIdTimeoutHandler class to include the word background as it essentially handles background session expiry.