peerstore : add GetPeersForProtocol API

[chaitanyaprem]

enhancement #1699

• Added new API in peerstore for GetPeersForProtocol.
• Implemented it for in memory peerstore.
• Ran regression tests for in memory peerstore with -race flag to ensure no new races are introduced.
• Add additional tests to validate new GetPeersForProtocol API

TODO:

• Implement the new API for DS based peerstore
• Run complete regression suite

[p-shahi]

Hi @chaitanyaprem we discussed this in the go-libp2p open maintainers call. We're thinking of a large refactor of the PeerStore, so we're hesitant to change the current api right now. Would you like to join our open maintainers call? It would be great to have you participate since you're taking on issues related to the PeerStore and we would like to have you in the discussion.
Here is the link to the community calendar which has the date & time for the maintainers call https://calendar.google.com/calendar/u/0/embed?src=libp2p.io_0q9682i3te7eanhe9q7ae1c58g@group.calendar.google.com

[chaitanyaprem]

ok @p-shahi . Will try to join this call from next week.
I have curently stopped working on this as implementing this in peerstoreDS is not very clean.
Wanted to wait for #2329 to get resolved.