server+addrmgr: add GetLastAttempt to fix race condition in connmgr

[Crypt-iQ]

Happens because ka.LastAttempt() is called without a mutex while the lastAttempt field is being updated by the addrmanager via Attempt. Can happen if two individual NewConnReq goroutines are spawned and are trying the same address.

==================
WARNING: DATA RACE
Write at 0x00c014f829d8 by goroutine 67:
  github.com/btcsuite/btcd/addrmgr.(*AddrManager).Attempt()
      /Users/nsa/go/src/github.com/btcsuite/btcd/addrmgr/addrmanager.go:861 +0x1b3
  main.newServer.func5()
      /Users/nsa/go/src/github.com/btcsuite/btcd/server.go:2822 +0x26f
  github.com/btcsuite/btcd/connmgr.(*ConnManager).NewConnReq()
      /Users/nsa/go/src/github.com/btcsuite/btcd/connmgr/connmanager.go:391 +0x3e6

Previous read at 0x00c014f829d8 by goroutine 69:
  main.newServer.func5()
      /Users/nsa/go/src/github.com/btcsuite/btcd/addrmgr/knownaddress.go:33 +0x498
  github.com/btcsuite/btcd/connmgr.(*ConnManager).NewConnReq()
      /Users/nsa/go/src/github.com/btcsuite/btcd/connmgr/connmanager.go:391 +0x3e6

Goroutine 67 (running) created at:
  github.com/btcsuite/btcd/connmgr.(*ConnManager).Start()
      /Users/nsa/go/src/github.com/btcsuite/btcd/connmgr/connmanager.go:530 +0x170

Goroutine 69 (running) created at:
  github.com/btcsuite/btcd/connmgr.(*ConnManager).Start()
      /Users/nsa/go/src/github.com/btcsuite/btcd/connmgr/connmanager.go:530 +0x170

[Roasbeef]

What about instead accepting the KnownAddress directly? Alternative, can we instead protect LastAttempt with a mutex?

[Crypt-iQ]

Both would work. Passing in KnownAddress would be easier since the addressmanager already has a mutex, but protecting LastAttempt with a mutex would be better stylistically. I think I'll do the latter

[Crypt-iQ]

I think I'll need to dig into the code a bit more. It seems that a lot of the members of KnownAddress can be racy, and the same goes for NetAddress. Think I need to figure out the best way to patch in one sweep.

[octobocto]

Has there been any progress on this?

[Crypt-iQ]

Still need to figure out all the fields that race

[jakesylvestre]

@jcvernaleo (as per #1530)

• high priority (anecdotally, I've run into this as well and it crashes the routine)
• bug