Adding table locking.

[jwoertink]

Fixes #594

This adds some options for doing table and row locks.

AppDatabase.with_lock_on(User, mode: :row_exclusive) do
  user = UserQuery.new.id(1).for_update.first
  SaveUser.update!(user, name: "New Name")
end

Testing it in a real app, and it seems to work... I'm not sure how to test this other than just ensuring the correct SQL is generated.

[jwoertink]

Postgres supports more than just FOR UPDATE

FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE }

So this might need some sort of args, or maybe just different methods like

def for_no_key_update
end
def for_share
end
def for_key_share
end

# VS

def for_update(for_method)
end
for_update(:no_key_update)
for_update(:share)