Don't download critical pieces before verification

This makes metadata always available before every request, and race
conditions between verification and downloading aren't possible.

Critical pieces are still prioritized for verification.

lib/torrent.js

@@ -605,7 +605,7 @@ Torrent.prototype._verify = function (index, cb) {
   var self = this
 
   if (self.destroyed) return cb(new Error('torrent is destroyed'))
-  if (self._verifying[index] || self.bitfield.get(index) || self._verified[index]) {
+  if (self._verifying[index] || self._verified[index]) {
     // call cb() asynchronously so that parallelLimit() won't block
     return process.nextTick(cb, null)
   }
@@ -614,8 +614,6 @@ Torrent.prototype._verify = function (index, cb) {
 
   self.store.get(index, function (err, buf) {
     if (self.destroyed) return cb(new Error('torrent is destroyed'))
-    // a critical piece might have been downloaded before this callback
-    if (self.bitfield.get(index)) return cb(null)
 
     if (err) {
       // interpret any error as not having the chunk
@@ -626,8 +624,6 @@ Torrent.prototype._verify = function (index, cb) {
 
     sha1(buf, function (hash) {
       if (self.destroyed) return cb(new Error('torrent is destroyed'))
-      // a critical piece might have been downloaded before this callback
-      if (self.bitfield.get(index)) return cb(null)
 
       if (hash === self._hashes[index]) {
         self._debug('piece from store verified %s', index)
@@ -1494,13 +1490,10 @@ Torrent.prototype._request = function (wire, index, hotswap) {
   if (self.bitfield.get(index)) return false
 
   if (!self._verified[index]) {
-    if (self._critical[index]) {
-      // if the piece is critical then start its verification immediately
-      self._verify(index, noop)
-    } else {
-      // cancel the request unless the piece is critical
-      return false
-    }
+    // verify critical pieces as soon as possible
+    if (self._critical[index]) self._verify(index, noop)
+
+    return false
   }
 
   var maxOutstandingRequests = isWebSeed
@@ -1531,11 +1524,7 @@ Torrent.prototype._request = function (wire, index, hotswap) {
   var chunkLength = isWebSeed ? piece.chunkLengthRemaining(reservation) : piece.chunkLength(reservation)
 
   wire.request(index, chunkOffset, chunkLength, function onChunk (err, chunk) {
-    // A critical piece may have been verified as in the store before the callback
-    if (self.destroyed || self.bitfield.get(index)) return
-
-    // TODO: what is this for?
-    if (!self.ready) return self.once('ready', function () { onChunk(err, chunk) })
+    if (self.destroyed) return
 
     if (r[i] === wire) r[i] = null