[SQL] Made Command.sideEffectResult protected

Considering `Command.executeCollect()` simply delegates to `Command.sideEffectResult`, we no longer need to leave the latter `protected[sql]`.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #2431 from liancheng/narrow-scope and squashes the following commits:

1bfc16a [Cheng Lian] Made Command.sideEffectResult protected