Revert "JCLOUDS-652: Require JDK 7"

This reverts commit 5e8bd02c93c7ebacb573342e77ef7ca4b02c4ffb.
Revert "Remove Java 6 litter"

This reverts commit a10bf3e9c5495b4c8c69e0059b5901266df471dd.
JCLOUDS-264 Conditionally allow default http driver to support sending >2GB payloads without chunked encoding.
JCLOUDS-652 Pin filesystem api to Java 7+.