Cleanup and promise concurrency.

[wraithgar]

This is a refactor done last year to clean up more of Arborist. I was
holding off submitting it till we could land
isaacs/promise-call-limit#12, since reify tests
randomly timed out for me without it.

Other PRs are now showing that behavior and solving the promise
concurrency / request timeout issue has now become something that cannot
wait. This PR can be where we update to that.

The diff that fixed it for me locally running off a linked version of
the promise-call-limit PR was:

diff --git a/workspaces/arborist/lib/arborist/reify.js b/workspaces/arborist/lib/arborist/reify.js
index 0981afdae..d07717984 100644
--- a/workspaces/arborist/lib/arborist/reify.js
+++ b/workspaces/arborist/lib/arborist/reify.js
@@ -31,6 +31,7 @@ const relpath = require('../relpath.js')
 const Diff = require('../diff.js')
 const retirePath = require('../retire-path.js')
 const promiseAllRejectLate = require('promise-all-reject-late')
+const promiseCallLimit = require('promise-call-limit')
 const optionalSet = require('../optional-set.js')
 const calcDepFlags = require('../calc-dep-flags.js')
 const { saveTypeMap, hasSubKey } = require('../add-rm-pkg-deps.js')
@@ -817,9 +818,11 @@ module.exports = cls => class Reifier extends cls {
     }

     // extract all the nodes with bundles
-    return promiseAllRejectLate(set.map(node => {
-      this[_bundleUnpacked].add(node)
-      return this[_reifyNode](node)
+    return promiseCallLimit(set.map(node => {
+      return () => {
+        this[_bundleUnpacked].add(node)
+        return this[_reifyNode](node)
+      }
     }))
     // then load their unpacked children and move into the ideal tree
       .then(nodes =>

(In that linked code I hard set rejectLate to true)