The binary cache store can now use HTTP/2 to do lookups. This is much
more efficient than HTTP/1.1 due to multiplexing: we can issue many
requests in parallel over a single TCP connection. Thus it's no longer
necessary to use a bunch of concurrent TCP connections (25 by
default).

For example, downloading 802 .narinfo files from
https://cache.nixos.org/, using a single TCP connection, takes 11.8s
with HTTP/1.1, but only 0.61s with HTTP/2.

This did require a fairly substantial rewrite of the Downloader class
to use the curl multi interface, because otherwise curl wouldn't be
able to do multiplexing for us. As a bonus, we get connection reuse
even with HTTP/1.1. All downloads are now handled by a single worker
thread. Clients call Downloader::enqueueDownload() to tell the worker
thread to start the download, getting a std::future to the result.

This allows readFile() to indicate that a file doesn't exist, and
might eliminate some large string copying.

Allowing stuff like

  NIX_REMOTE=https://cache.nixos.org nix-store -qR /nix/store/x1p1gl3a4kkz5ci0nfbayjqlqmczp1kq-geeqie-1.1

or

  NIX_REMOTE=https://cache.nixos.org nix-store --export /nix/store/x1p1gl3a4kkz5ci0nfbayjqlqmczp1kq-geeqie-1.1 | nix-store --import

Allowing stuff like

  NIX_REMOTE=https://cache.nixos.org nix-store -qR /nix/store/x1p1gl3a4kkz5ci0nfbayjqlqmczp1kq-geeqie-1.1

or

  NIX_REMOTE=https://cache.nixos.org nix-store --export /nix/store/x1p1gl3a4kkz5ci0nfbayjqlqmczp1kq-geeqie-1.1 | nix-store --import

Also, move a few free-standing functions into StoreAPI and Derivation.

Also, introduce a non-nullable smart pointer, ref<T>, which is just a
wrapper around std::shared_ptr ensuring that the pointer is never
null. (For reference-counted values, this is better than passing a
"T&", because the latter doesn't maintain the refcount. Usually, the
caller will have a shared_ptr keeping the value alive, but that's not
always the case, e.g., when passing a reference to a std::thread via
std::bind.)

This makes it consistent with the Nixpkgs fetchurl and makes it work
in chroots. We don't need verification because the hash of the result
is checked anyway.

This allows overriding the name component of the resulting Nix store
path, which is necessary if the base name of the URI contains
"illegal" characters.

This is in particular useful for fetchFromGitHub et al., ensuring that
the store path produced by nix-prefetch-url corresponds to what those
functions expect.

This allows nix-prefetch-url to prefetch the output of fetchzip and
its wrappers (like fetchFromGitHub). For example:

  $ nix-prefetch-url --unpack https://github.com/NixOS/patchelf/archive/0.8.tar.gz

or from a Nix expression:

  $ nix-prefetch-url -A nix-repl.src

In the latter case, --unpack can be omitted because nix-repl.src is a
fetchFromGitHub derivation and thus has "outputHashMode" set to
"recursive".

For example,

  $ nix-prefetch-url -A hello.src

will prefetch the file specified by the fetchurl call in the attribute
‘hello.src’ from the Nix expression in the current directory. This
differs from ‘nix-build -A hello.src’ in that it doesn't verify the
hash.

You can also specify a path to the Nix expression:

  $ nix-prefetch-url ~/Dev/nixpkgs -A hello.src

List elements (typically used in ‘patches’ attributes) also work:

  $ nix-prefetch-url -A portmidi.patches.0