[#1836] Add comment to explain why read is triggered
More efficient handling of incomplete writes.

The problem with the old way was that we always set the OP_WRITE when the buffer could not be written
until the write-spin-count was reached. This means that in some cases the channel was still be writable
but we just was not able to write out the data quick enough. For this cases we should better break out the
write loop and schedule a write to be picked up later in the EventLoop, when other tasks was executed.
The OP_WRITE will only be set if a write actual returned 0 which means there is no more room for writing data
and this we need to wait for the os to notify us.
[#1852] Fix bug in UnpooledDirectByteBuf.nioBuffer(...) implementation
[#1853] Optimize gathering writes for CompositeByteBuf that are only backed by one ByteBuffer
[#1844] HttpRequestEncoder should append '/' to absolute path requests only when needed