Use fork of fibers package that triggers GC less often.

Apparently calling v8::Isolate::AdjustAmountOfExternalAllocatedMemory
frequently results in lots of wasted CPU cycles on garbage collection, per
discussion here: meteor#10527 (comment)

This fix was inspired by marudor/libxmljs2#22, which seems to have
addressed nodejs/node#30995.

Another project that benefitted from adjusting external allocated memory
less often: mapnik/node-mapnik#136

scripts/dev-bundle-server-package.js

@@ -10,7 +10,7 @@ var packageJson = {
   dependencies: {
     // Keep the versions of these packages consistent with the versions
     // found in dev-bundle-tool-package.js.
-    fibers: "4.0.2",
+    fibers: "https://github.com/meteor/node-fibers/tarball/df2c3f135f0f21bfd799eb7f8d33740c0cee6688",
     "meteor-promise": "0.8.7",
     promise: "8.0.2",
     reify: "0.20.12",

scripts/dev-bundle-tool-package.js

@@ -19,7 +19,7 @@ var packageJson = {
     // Keep the versions of these packages consistent with the versions
     // found in dev-bundle-server-package.js.
     "meteor-promise": "0.8.7",
-    fibers: "4.0.2",
+    fibers: "https://github.com/meteor/node-fibers/tarball/df2c3f135f0f21bfd799eb7f8d33740c0cee6688",
     reify: "0.20.12",
     // So that Babel can emit require("@babel/runtime/helpers/...") calls.
     "@babel/runtime": "7.7.6",