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Encapsulate functions for readability, and move code for understanding

Move the job of splitting requests to its own package

As soon as peers appear, consume all of the want budget

Reduce duplicates through splits of requests

fix #43

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