[NWD] More EPPlus improvements. - Revamp save code to avoid memory copies - Remove more unnecessary code for mutating sheets - General cleanup Change-Id: I9faf6c7cc94a50600076bd42ac456ba6f96cd407 Reviewed-on: https://gnocchi-internal-review.git.corp.google.com/c/third_party/epplus/+/208000 Reviewed-by: Hughes Hilton <hugheshilton@google.com>