[ipfs/go-bitswap] Exposing the session peers

[hannahhoward]

What

The session peers are super useful, and also, they aren't publicly available.

It would really be great to expose them outside of bitswap.

It would be even better to extract session peer management entirely, so other transports could write peers into the library

[burdiyan]

Spent a few hours digging through the internals to see if session peers are exposed in any way and realized they weren't :(