fix: make select...for update locks be a no-op on cockroachdb

In Cockroachdb there is no select for update
(see cockroachdb/cockroach#6583)
But it is not needed, make it a no-op.

lib/core_ext/active_record/connection_adapters/cockroachdb/lock.rb

@@ -0,0 +1,12 @@
+# Cockroachdb does not implement the locking of select...for update.
+# but, it doesn't need it since it uses serialised updates. So
+# we make it a no-op
+# https://github.com/cockroachdb/activerecord-cockroachdb-adapter/issues/21
+module ActiveRecord
+  class Base
+    def lock!(lock = true)
+      super(lock) unless self.class.connection.class.to_s == 'ActiveRecord::ConnectionAdapters::CockroachDBAdapter'
+    end
+  end
+end
+