util: New macros ROUND_UP_POW2, ROUND_DOWN_POW2.

Signed-off-by: Ben Pfaff <blp@nicira.com>
async-append: New library to allow asynchronous appending to a log file.

This will be hooked into the vlog library in an upcoming commit.

Signed-off-by: Ben Pfaff <blp@nicira.com>
vlog: Use async I/O.

Signed-off-by: Ben Pfaff <blp@nicira.com>
vlog: Make thread-safe.

Signed-off-by: Ben Pfaff <blp@nicira.com>