Motivation:
The API documentation in ChannelConfig states that a a channel is writable,
if the number of pending bytes is below the low watermark and a
channel is not writable, if the number of pending bytes exceeds the high
watermark.
Therefore, we should use < operators instead of <= as well as > instead of >=.
Using <= and >= is also problematic, if the low watermark is equal to the high watermark,
as then a channel could be both writable and unwritable with the same number of pending
bytes (depending on whether remove() or addMessage() is called first).
The use of <= and >= was introduced in PR https://github.com/netty/netty/pull/3036, but
I don't understand why, as there doesn't seem to have been any discussion around that.
Modifications:
Use < and > operators instead of <= and >=.
Result:
High and low watermarks are treated as stated in the API docs.
d79e5c594d