Detecting client disconnection (yhirose#1373)

* SocketStream need to check connectivity for writability.

When the stream is used for SSE server which works via chunked content
provider it's not possible to check connectivity over writability
because it's wrapped by DataSink. It could make the server stalled, when
the provider wants to only keep connection without send data and certain
amount of clients consumes entire threadpool.

* add unittest for SocketStream::is_writable()

SocketStream::is_writable() should return false if peer is disconnected.

* revise broken unittest ServerTest.ClientStop

DataSink could be unwritable if it's backed by connection based. Because
it could be disconnected by client right after enter centent provider.

Co-authored-by: Changbin Park <[email protected]>

Author: gh4ck3r