Add store update methods

[palkan]

Examples:

1. Delete hstore keys:

# before
Post.update_all(["labels = delete(labels, ARRAY[?])", id.to_s])

# after
Post.update_store(:labels).delete_keys(id)

2. Merge hstore

# before
Post.update_all(["labels = hstore(labels) || hstore(ARRAY[:keys])",
                   keys: labels.map { |g, l| [g.to_s, l.to_s] }.flatten])

# after
Post.update_store(:labels).merge(labels)

3. Delete pairs

Post.update_store(:labels).delete_pairs(labels)