storage.in_memory: Add person_get implementation
Implement person_get for the in-memory storage.
I have also updated the person counter in the stat_counters method.
This is needed to remove all the storage mocks in swh-web tests.
Related swh-web#1271 (closed) Related #1377 (closed)
Test Plan
test_person_get is now executed for the in-memory storage tests.
Migrated from D897 (view on Phabricator)