Assign one store per peer in `ConvexTest`

[helins]

I noticed all peers are initialized with the same store. I would strongly suggest using 1 dedicated store per peer. Otherwise it masks potential MissingDataExceptions.

[mikera]

Ideally we should test both with separate stores and a shared store. We do want to test concurrent shared store usage as well!

[mikera]

Double check that Peers being tested with independent stores as well as shared stores