FilesystemStoreV2 already rejected v1 data when a key file was found
at the store root, but it did not inspect namespace directories. This
missed v1 layouts such as primary/key, where v2 expects primary to
contain secondary namespace directories.

For example, an ldk-node store can contain a BDK descriptor below a
namespace directory. The previous check would accept that directory as
v2 data because the root contained only directories, leaving the
incompatible descriptor file undetected.

Backport of 668b35b

Backport 4649 to 0.3

Prepare the migration API naming for an async variant by giving the
existing synchronous trait an explicit Sync suffix.

Backport of 4060b62

Co-Authored-By: HAL 9000

Expose an async migratable KV store trait and async migration helper so
async stores can migrate data without using the synchronous API.

Backport of 9b11fda

Co-Authored-By: HAL 9000

Allow the filesystem stores to use the async migration helper and cover
both store versions with async migration tests.

Backport of 94cff3b

Co-Authored-By: HAL 9000

…-0.3

Add async variant of `MigratableKVStore` (0.3 backport)