refactor: put mutex next to the things it protects

If we put the lock next to the fields it protects, it can sometimes make
it easier to reason about threadsafety.

In this case, it reveals that the task queue (not threadsafe) isn't protected by the
mutex, yet shared between the worker and callers.

@whyrusleeping

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>

exchange/bitswap/strategy/ledgermanager.go

@@ -22,15 +22,19 @@ type Envelope struct {
 }
 
 type LedgerManager struct {
-	lock sync.RWMutex
-	// ledgerMap lists Ledgers by their Partner key.
-	ledgerMap map[u.Key]*ledger
-	bs        bstore.Blockstore
 	// FIXME taskqueue isn't threadsafe nor is it protected by a mutex. consider
 	// a way to avoid sharing the taskqueue between the worker and the receiver
-	taskqueue  *taskQueue
-	outbox     chan Envelope
+	taskqueue *taskQueue
+
 	workSignal chan struct{}
+
+	outbox chan Envelope
+
+	bs bstore.Blockstore
+
+	lock sync.RWMutex
+	// ledgerMap lists Ledgers by their Partner key.
+	ledgerMap map[u.Key]*ledger
 }
 
 func NewLedgerManager(ctx context.Context, bs bstore.Blockstore) *LedgerManager {