async-append: Refactor to avoid requiring enabling while single threaded.

async-append: Refactor to avoid requiring enabling while single threaded.

Until now, the async append interface has required async_append_enable()
to be called while the process was still single-threaded, with the
rationale being that async_append_enable() could race with
async_append_write() on some existing async_append object.  This was a
difficult problem when the async append interface was introduced, because
at the time Open vSwitch did not have any infrastructure for inter-thread
synchronization.

Now it is easy to solve, by introducing synchronization into the
async append module.  However, that's more or less wasted, because the
client is already required to serialize access to async append objects.
Moreover, vlog, the only existing client, needs to serialize access for
other reasons, so it wouldn't even be possible to just drop the client's
synchronization.

This commit therefore takes another approach.  It drops the
async_append_enable() interface entirely.  Now any existing async_append
object is always enabled.  The responsibility for "enabling", then, now
rests in whether the client creates and uses an async_append object, and
so vlog now takes care of that by itself.  Also, since vlog now has to
deal with sometimes having an async_append and sometimes not having one,
we might as well allow creating an async_append to fail, thereby slightly
simplifying the "no async I/O" implementation from "write synchronously"
to "always fail creating an async_append".

Reported-by: Shih-Hao Li <shihli@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>