fix: add lock to taskQueue

@whyrusleeping may wanna have a look and make sure i didn't screw
anything up here

BenchmarkInstantaneousAddCat1MB-4            200          10763761 ns/op
97.42 MB/s
BenchmarkInstantaneousAddCat2MB-4       panic: runtime error: invalid
memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0xbedd]

goroutine 14297 [running]:
github.com/jbenet/go-ipfs/exchange/bitswap/decision.(*taskQueue).Remove(0xc2087553a0,
        0xc2085ef200, 0x22, 0x56f570, 0xc208367a40)
    /Users/btc/go/src/github.com/jbenet/go-ipfs/exchange/bitswap/decision/taskqueue.go:66
    +0x82
github.com/jbenet/go-ipfs/exchange/bitswap/decision.(*Engine).MessageSent(0xc20871b5c0,
        0x56f570, 0xc208367a40, 0x570040, 0xc208753d40, 0x0, 0x0)
    /Users/btc/go/src/github.com/jbenet/go-ipfs/exchange/bitswap/decision/engine.go:177
    +0x29e
github.com/jbenet/go-ipfs/exchange/bitswap.(*bitswap).send(0xc20871b7a0,
        0x56f4d8, 0xc208379800, 0x56f570, 0xc208367a40,
        0x570040, 0xc208753d40, 0x0, 0x0)
    /Users/btc/go/src/github.com/jbenet/go-ipfs/exchange/bitswap/bitswap.go:352
    +0x11c
github.com/jbenet/go-ipfs/exchange/bitswap.(*bitswap).taskWorker(0xc20871b7a0,
        0x56f4d8, 0xc208379800)
    /Users/btc/go/src/github.com/jbenet/go-ipfs/exchange/bitswap/bitswap.go:238
    +0x165
    created by
    github.com/jbenet/go-ipfs/exchange/bitswap.New
    /Users/btc/go/src/github.com/jbenet/go-ipfs/exchange/bitswap/bitswap.go:66
    +0x49e

decision/taskqueue.go

 package decision
 
 import (
+	"sync"
+
 	wantlist "github.com/jbenet/go-ipfs/exchange/bitswap/wantlist"
 	peer "github.com/jbenet/go-ipfs/peer"
 	u "github.com/jbenet/go-ipfs/util"
@@ -11,6 +13,7 @@ import (
 // tasks (on getnext). For now, we are assuming a dumb/nice strategy.
 type taskQueue struct {
 	// TODO: make this into a priority queue
+	lock    sync.Mutex
 	tasks   []*task
 	taskmap map[string]*task
 }
@@ -29,6 +32,8 @@ type task struct {
 
 // Push currently adds a new task to the end of the list
 func (tl *taskQueue) Push(entry wantlist.Entry, to peer.Peer) {
+	tl.lock.Lock()
+	defer tl.lock.Unlock()
 	if task, ok := tl.taskmap[taskKey(to, entry.Key)]; ok {
 		// TODO: when priority queue is implemented,
 		//       rearrange this task
@@ -45,6 +50,8 @@ func (tl *taskQueue) Push(entry wantlist.Entry, to peer.Peer) {
 
 // Pop 'pops' the next task to be performed. Returns nil no task exists.
 func (tl *taskQueue) Pop() *task {
+	tl.lock.Lock()
+	defer tl.lock.Unlock()
 	var out *task
 	for len(tl.tasks) > 0 {
 		// TODO: instead of zero, use exponential distribution
@@ -63,10 +70,12 @@ func (tl *taskQueue) Pop() *task {
 
 // Remove lazily removes a task from the queue
 func (tl *taskQueue) Remove(k u.Key, p peer.Peer) {
+	tl.lock.Lock()
 	t, ok := tl.taskmap[taskKey(p, k)]
 	if ok {
 		t.Trash = true
 	}
+	tl.lock.Unlock()
 }
 
 // taskKey returns a key that uniquely identifies a task.