Lose the boost dependency since recent versions of capnproto's kj
also provide a nice filesystem library. Take the opportunity to
refactor the Run object to become more than POD and to encapsulate
some of the functionality that was done in the Laminar class
Part of #49 refactor
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
