Bug 63657: Rework the for bug #62130 to not use up twice as much memory when writing...

Bug 63657: Rework the for bug #62130 to not use up twice as much memory when writing documents.

Unfortunately XMLBeans is very tricky to use here, mainly the fact that setCArray does not replace the internal objects, but copies the content into the currently held objects makes it rather hard to do this right.

Therefore we now try to keep the existing objects and only replace the content as required to
have a stable ordering of cells in the row-XML structure.

This also fixes removing cells from rows to avoid invalid situations and
correctly free CTCellImpl instances.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1864977 13f79535-47bb-0310-9956-ffa450edef68