Cache ProfileStorageBase.get_this_revision

Previously measured during `clive show profiles`:

```
get_this_revision for <class 'clive.__private.storage.migrations.v0.ProfileStorageModel'> took 0.013s.
get_this_revision for <class 'clive.__private.storage.migrations.v0.ProfileStorageModel'> took 0.007s.
get_this_revision for <class 'clive.__private.storage.migrations.v0.ProfileStorageModel'> took 0.008s.
get_this_revision for <class 'clive.__private.storage.migrations.v1.ProfileStorageModel'> took 0.007s.
get_this_revision for <class 'clive.__private.storage.migrations.v1.ProfileStorageModel'> took 0.007s.
get_this_revision for <class 'clive.__private.storage.migrations.v1.ProfileStorageModel'> took 0.007s.
get_this_revision for <class 'clive.__private.storage.migrations.v2.ProfileStorageModel'> took 0.007s.
get_this_revision for <class 'clive.__private.storage.migrations.v2.ProfileStorageModel'> took 0.007s.
get_this_revision for <class 'clive.__private.storage.migrations.v2.ProfileStorageModel'> took 0.007s.
```

Now:

```
get_this_revision for <class 'clive.__private.storage.migrations.v0.ProfileStorageModel'> took 0.013s.
get_this_revision for <class 'clive.__private.storage.migrations.v1.ProfileStorageModel'> took 0.008s.
get_this_revision for <class 'clive.__private.storage.migrations.v2.ProfileStorageModel'> took 0.007s.
```