server: Fix syncNotified race.

[dnldd]

This fixes a race with the syncNotified field of a serverPeer.

==================                                                                                                                                                                                          
WARNING: DATA RACE                                                                                                                                                                                          
Read at 0x00c01dff946b by goroutine 183:                                                                                                                                                                    
  main.(*server).peerDoneHandler()                                                                                                                                                                          
      /Users/dnldd/Work/dcrd/server.go:2253 +0xa8                                                                                                                                                           
                                                                                                                                                                                                            
Previous write at 0x00c01dff946b by goroutine 46:                                                                                                                                                           
  main.(*server).peerHandler()                                                                                                                                                                              
      /Users/dnldd/Work/dcrd/server.go:2317 +0x4b4                                                                                                                                                          
                                                                                                                                                                                                            
Goroutine 183 (running) created at:                                                                                                                                                                         
  main.(*server).outboundPeerConnected()                                                                                                                                                                    
      /Users/dnldd/Work/dcrd/server.go:2236 +0x5bc                                                                                                                                                          
  main.(*server).outboundPeerConnected-fm()                                                                                                                                                                 
      /Users/dnldd/Work/dcrd/server.go:2224 +0x58
...