Fixes a crash that happens on g++ 6.3 under debian but not
g++ 7.2.1 under arch. Instead of trying to move kj::AsyncIoStream
between each promise handler, just extract and use the raw
pointer and let attach() clean up the Own<> at the end
Read or writes to bad file descriptors threw exceptions, just log
this instead. Also make sure that LaminarClients are removed from
the list of clients when a connection is aggressively dropped, and
add test
