remove the database-backed peerstore

As far as I can tell, it's only used by drand in PLN: https://github.com/drand/drand/blob/ade43023d83b5b06f15f6563ec0e7b246b0829db/lp2p/ctor.go#L21
And by 0xmesh: https://github.com/0xProject/0x-mesh/blob/a40744bd096d2888fc16941bb940959254ea7c38/p2p/opts.go#L18
(There might be more)

The database-backed peerstore is what required https://github.com/libp2p/go-libp2p/issues/2231 and the mess we got into by adding contexts (see #2327).

It doesn't make a lot of sense to have a database-backed peerstore in the first place. The peerstore uses the disconnect event to garbage collect old entries, so there's little point in persisting this information. You'd just end up with entries that will never be evicted.

Note that this proposal is only to remove our pstoreds implementation of the peerstore interfaces. Users that still believe that there's value in having a database-backed peerstore are free to adopt that code: Applications can provide their own implementation of the peerstore interface in the libp2p constructor.

Start by locating the pstoreds peerstore implementation and all references to it, including the drand and 0x-mesh examples linked in the issue. Check the peerstore interfaces and related tests before removing the implementation; done means no supported libp2p code depends on pstoreds and the remaining test suite passes.