Move to boost::asio::post

This allows entity_manager to compile with BOOST_ASIO_NO_DEPRECATED set.
It was functionally changed a few years ago, and is identical to the
other behavior.

Change-Id: Ib8125fcf9bdddf0bafd46118c4e764f7f83a0c2d
Signed-off-by: Ed Tanous <edtanous@google.com>
1 file changed