Migrate LFS shas in database from string to binary

What

Change schema and migrate data for LfsObject, LfsPointer, etc

Why

Suggested by @yorickpeterse in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14479#note_42617056:

Also since OIDs are SHA1s IIRC we should use binary here and use the ShaAttribute module to store the data more efficiently.