Fixes https://github.com/tendermint/tendermint/issues/851

Go1.9 and below's net.Pipe did not implement the SetDeadline
method so after commit
e2dd8ca946
this problem was exposed since now we check for errors.

To counter this problem, implement a simple composition for
net.Conn that always returns nil on SetDeadline instead of
tripping out.

Added build tags so that anyone using go1.10 when it is released
will be able to automatically use net.Pipe's net.Conns

Noticed while auditing the code that we aren't respecting
(*net.Conn) SetDeadline errors which return after
a connection has been killed and is simultaneously
being used.

For example given program, without SetDeadline error checks
```go
package main

import (
  "log"
  "net"
  "time"
)

func main() {
  conn, err := net.Dial("tcp", "tendermint.com:443")
  if err != nil {
    log.Fatal(err)
  }
  go func() {
    <-time.After(400 * time.Millisecond)
    conn.Close()
  }()
  for i := 0; i < 5; i++ {
    if err := conn.SetDeadline(time.Now().Add(time.Duration(10 * time.Second))); err != nil {
      log.Fatalf("set deadline #%d, err: %v", i, err)
    }
    log.Printf("Successfully set deadline #%d", i)
    <-time.After(150 * time.Millisecond)
  }
}
```

erraneously gives
```shell
2017/11/14 17:46:28 Successfully set deadline #0
2017/11/14 17:46:29 Successfully set deadline #1
2017/11/14 17:46:29 Successfully set deadline #2
2017/11/14 17:46:29 Successfully set deadline #3
2017/11/14 17:46:29 Successfully set deadline #4
```

However, if we properly fix it to respect that error with
```diff
--- wild.go 2017-11-14 17:44:38.000000000 -0700
+++ main.go 2017-11-14 17:45:40.000000000 -0700
@@ -16,7 +16,9 @@
    conn.Close()
  }()
  for i := 0; i < 5; i++ {
-   conn.SetDeadline(time.Now().Add(time.Duration(10 * time.Second)))
+   if err := conn.SetDeadline(time.Now().Add(time.Duration(10 *
time.Second))); err != nil {
+     log.Fatalf("set deadline #%d, err: %v", i, err)
+   }
    log.Printf("Successfully set deadline #%d", i)
    <-time.After(150 * time.Millisecond)
  }
```

properly catches any problems and gives
```shell
$ go run main.go
2017/11/14 17:43:44 Successfully set deadline #0
2017/11/14 17:43:45 Successfully set deadline #1
2017/11/14 17:43:45 Successfully set deadline #2
2017/11/14 17:43:45 set deadline #3, err: set tcp 10.182.253.51:57395:
use of closed network connection
exit status 1
```

Just noticed while auditing the code in p2p/addrbook.go,
wg.Add(1) but no subsequent defer.
@jaekwon and I had a discussion offline and we agreed to
comment about why the code was that way and why
we shouldn't move the wg.Add(1) into .saveRoutine() because
if go a.saveRoutine() isn't started before anyone invokes
a.Wait(), then we'd have raced a.saveRoutine().

and a test for it

Fixes #602

Delete unused and untested functions:
- AddToIPRangeCounts
- CheckIPRangeCounts

* Full test PeerSet and check its concurrent guarantees
* Improve the doc for PeerSet.Has and remove unnecessary
defer for a path that sets a variable, make it fast anyways.
* Parallelize PeerSet tests with t.Parallel()
* Document functions in peer_set.go more.

Updates #628

Signed-off-by: Adrian Brink <adrian@brink-holdings.com>

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x882cec]

goroutine 328 [running]:
github.com/tendermint/tendermint/p2p.(*Switch).DialPeerWithAddress(0xc42000a500, 0xc4202088d0, 0xc420403500, 0x0, 0x0, 0x0)
        /home/vagrant/go/src/github.com/tendermint/tendermint/p2p/switch.go:324 +0x2fc
github.com/tendermint/tendermint/p2p.(*PEXReactor).ensurePeers.func1(0xc4201663f0, 0xc4202088d0)
        /home/vagrant/go/src/github.com/tendermint/tendermint/p2p/pex_reactor.go:280 +0x3e
created by github.com/tendermint/tendermint/p2p.(*PEXReactor).ensurePeers
        /home/vagrant/go/src/github.com/tendermint/tendermint/p2p/pex_reactor.go:284 +0x5d4
```