iochannel: Avoid unnecessary wakeup after successful write
To save some CPU (in low latency scenarios), don't re-enable the
"writable" event after it has succeeded. It is very likely the next
write will succeed right away too.
This means that we always need to handle EAGAIN/EWOULDBLOCK as a
successful write of 0 bytes, so I also verified that all callers to
pa_iochannel_write handled this correctly.
Signed-off-by: David Henningsson <david.henningsson@canonical.com>