Optimize testenv-rebuild initial rebuild

When running testenv-rebuild for the first time in a testenv it is likely the list of rsync'd files will include files where the contents has not really changed but the files have different (newer) mtimes. The most likely reason is git - when switching branches it updates mtimes for files with changes. If these false positives include *.yang files the build script will execute a full packages reload at the end. This is a time-consuming operation when many packages are loaded and is unnecessary.

To avoid this we run the first rsync with checksumming instead, then after it is done and for the rest of the container lifetime we run it in default (mtime + size comparison) mode.

Closes #273 (closed)