GitLab

Add a unit test for session package

extract the job of finding and managing peers for a session from the job of requesting blocks

remove for loop not needed, cleanup spelling

- moved sessions out of main bitswap package
- modified session manager to manage all sessions
- moved get functions to their own package so sessions can directly

BREAKING CHANGE: SessionsForBlock, while not used outside of Bitswap, has been removed, and was an
exported function