feat(store/v2): Assume SS/SC backends are concurrent by default

[testinginprod]

In order to simplify the store package, we should assume all SS/SC backends are thread safe by default, the implementation should handle concurrency itself in such a way to achieve the best lock granularity.

[alexanderbez]

The default root store implementation uses a mutex lock. What exactly is the rationale for the underlying SS backends to have additional locks? It seems to me that we should rely on the storage engine's concurrency mechanism.

[testinginprod]

@alexanderbez this was already addressed in: https://github.com/cosmos/cosmos-sdk/pull/17995/files#diff-1d3af25f440ffa070f28b311f269f2ed08b17dc88847a2768f727692930926a5L1

closing.