Liburing 2.8-rc1 pre-release

liburing-2.8 pre-release

• Add support for incrementally/partially consumed provided buffers,
  usable with the provided buffer ring support.
• Add support for foo_and_wait_min_timeout(), where it's possible to
  define a minimum timeout for waiting to get batches of completions,
  but if that fails, extend for a longer timeout without having any
  extra context switches.
• Add support for using different clock sources for completion waiting.
• Great increase coverage of test cases, test case improvements and
  fixes.
• Man page updates
• Don't leak _GNU_SOURCE via pkb-config --cflags
• Support for address sanitizer
• Add examples/kdigest sample program