Queue thread safety

[iHiroakiKawai]

files under src/lib should not depend on src/switch and mutex routines are placed under src/switch.

This patch is puts mutex under src/switch side.

[iHiroakiKawai]

another option is #126

[yasuhito]

@iHiroakiKawai ありがとうございます! #126 をマージしたのでこちらは閉じておきます。