REORG: channel: Rename CF_WRITE_NULL to CF_WRITE_EVENT

As for CF_READ_NULL, it appears CF_WRITE_NULL and other write events on a
channel are mainly used to wake up the stream and may be replace by on write
event.

In this patch, we introduce CF_WRITE_EVENT flag as a replacement to
CF_WRITE_EVENT_NULL. There is no breaking change for now, it is just a
rename. Gradually, other write events will be merged with this one.
4 files changed