core: concurrent database reinit from freezer dump

* core: reinit chain from freezer in batches

* core/rawdb: concurrent database reinit from freezer dump

* core/rawdb: reinit from freezer in sequential order

common/prque/prque.go

@@ -1,5 +1,20 @@
+// CookieJar - A contestant's algorithm toolbox
+// Copyright (c) 2013 Peter Szilagyi. All rights reserved.
+//
+// CookieJar is dual licensed: use of this source code is governed by a BSD
+// license that can be found in the LICENSE file. Alternatively, the CookieJar
+// toolbox may be used in accordance with the terms and conditions contained
+// in a signed written agreement between you and the author(s).
+
 // This is a duplicated and slightly modified version of "gopkg.in/karalabe/cookiejar.v2/collections/prque".
 
+// Package prque implements a priority queue data structure supporting arbitrary
+// value types and int64 priorities.
+//
+// If you would like to use a min-priority queue, simply negate the priorities.
+//
+// Internally the queue is based on the standard heap package working on a
+// sortable version of the block based stack.
 package prque
 
 import (
@@ -11,8 +26,8 @@ type Prque struct {
 	cont *sstack
 }
 
-// Creates a new priority queue.
-func New(setIndex setIndexCallback) *Prque {
+// New creates a new priority queue.
+func New(setIndex SetIndexCallback) *Prque {
 	return &Prque{newSstack(setIndex)}
 }
 
@@ -21,6 +36,12 @@ func (p *Prque) Push(data interface{}, priority int64) {
 	heap.Push(p.cont, &item{data, priority})
 }
 
+// Peek returns the value with the greates priority but does not pop it off.
+func (p *Prque) Peek() (interface{}, int64) {
+	item := p.cont.blocks[0][0]
+	return item.value, item.priority
+}
+
 // Pops the value with the greates priority off the stack and returns it.
 // Currently no shrinking is done.
 func (p *Prque) Pop() (interface{}, int64) {