Undertow does not provide a way to check if we can write so with the
current implementation of isWritePossible, deep recursion can occur
when writing slows down. We now use a flag to keep track of write
ChannelListener callbacks.
This commit also addresses a related issue in
AbstractListenerWriteProcessor that went undected since #3c2d186
where after a large (single) buffer that is not written fully, the
completion signal is processed before the all data is written.
Issue: SPR-16702
3549745e37