Enable HTTP/2 support

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.

src/libexpr/common-opts.cc

@@ -55,7 +55,7 @@ bool parseSearchPathArg(Strings::iterator & i,
 Path lookupFileArg(EvalState & state, string s)
 {
     if (isUri(s))
-        return makeDownloader()->downloadCached(state.store, s, true);
+        return getDownloader()->downloadCached(state.store, s, true);
     else if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
         Path p = s.substr(1, s.size() - 2);
         return state.findFile(p);

src/libexpr/parser.y

@@ -662,7 +662,7 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
                 // FIXME: support specifying revision/branch
                 res = { true, exportGit(store, elem.second, "master") };
             else
-                res = { true, makeDownloader()->downloadCached(store, elem.second, true) };
+                res = { true, getDownloader()->downloadCached(store, elem.second, true) };
         } catch (DownloadError & e) {
             printMsg(lvlError, format("warning: Nix search path entry ‘%1%’ cannot be downloaded, ignoring") % elem.second);
             res = { false, "" };

src/libexpr/primops.cc

@@ -1769,7 +1769,7 @@ void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
     if (state.restricted && !expectedHash)
         throw Error(format("‘%1%’ is not allowed in restricted mode") % who);
 
-    Path res = makeDownloader()->downloadCached(state.store, url, unpack, name, expectedHash);
+    Path res = getDownloader()->downloadCached(state.store, url, unpack, name, expectedHash);
     mkString(v, res, PathSet({res}));
 }