When finding the CTCol object for a XSSF Sheet column, as we often have to traverse most of the list, and we are mostly reading not changing, switch to fetching all the objects as an array in one go, rather than fetching one at a time, which is hopefully faster. Aims to help bug #56556

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1600319 13f79535-47bb-0310-9956-ffa450edef68