vlog: Use worker process to write to log file.

Writes to regular files under Unix-like kernels, including Linux, typically
block until the write is complete, regardless of O_NONBLOCK.  When the I/O
subsystem is busy, this can cause indefinite delays.  We have actually
observed "write" calls sleep for 5 seconds or more for this reason.

Delegating to a subprocess through the worker mechanism should solve the
problem.

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