The old implementation slurped the whole artefact into memory, and
did not ensure it remained allocated beyond the first call to write().
The new implementation uses mmap and ensures the mapping lasts until
the file has been delivered
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
