[boost] [1.50.0] Release branch open for bug fixes and doc updates

The final release is scheduled for Monday, June 25th. The release candidate is scheduled to to be built Thursday, June 21st. That gives us two weeks to apply post-beta bug fixes and doc updates. Let's try a slightly more streamlined procedure for this post-beta period; until Monday, June 18th permission is not required to merge bug fixes and documentation updates. Starting June 18th permission of a release manager is required for all merges. As always, New libraries and new features are not allowed during a post-beta period. Thanks, --Beman