[net] Make p2p recv buffer timeout 20 minutes for all peers

The timeout interval for the send and recv buffers was changed from 90
minutes to 20 minutes in commit f1920e86 in 2013, except for peers that
did not support the pong message (where the recv buffer timeout remained
at 90 minutes). A few observations:

- for peers that support BIP 31 (pong messages), this recv buffer
  timeout is almost redundant with the ping timeout. We send a ping
  message every two minutes, and set a timeout of twenty minutes to
  receive the pong response. If the recv buffer was really timing out,
  then the pong response would also time out.
- BIP 31 is supported by all nodes of p2p version 60000 and higher, and
  has been in widespread use since 2013. I'd be very surprised if there
  are many nodes on the network that don't support pong messages.
- The recv buffer timeout is not specified in any p2p BIP. We're free to
  set it at any value we want.
- A peer that doesn't support BIP 31 and hasn't sent any message to us
  at all in 90 minutes is unlikely to be useful for us, and is more likely
  to be evicted AttemptToEvictConnection() since it'll have the worst
  possible ping time and isn't providing blocks/transactions.

Therefore, we remove this check, and sent the recv buffer timeout to 20
minutes for all peers. This removes the final p2p version dependent
logic from the net layer, so all p2p version data can move into the
net_processing layer.

Alternative approaches:

- Set the recv buffer timeout to 90 minutes for all peers. This almost
  wouldn't be a behaviour change at all (pre-BIP 31 peers would still
  have the same recv buffer timeout, and we can't ever reach a recv buffer
  timeout higher than 21 minutes for post-BIP31 peers, because the pong
  timeout would be hit first).
- Stop supporting peers that don't support BIP 31. BIP 31 has been in
  use since 2012, and implementing it is trivial.

src/net.cpp

@@ -1232,7 +1232,7 @@ void CConnman::InactivityCheck(CNode *pnode)
             LogPrintf("socket sending timeout: %is\n", nTime - pnode->nLastSend);
             pnode->fDisconnect = true;
         }
-        else if (nTime - pnode->nLastRecv > (pnode->GetCommonVersion() > BIP0031_VERSION ? TIMEOUT_INTERVAL : 90*60))
+        else if (nTime - pnode->nLastRecv > TIMEOUT_INTERVAL)
         {
             LogPrintf("socket receive timeout: %is\n", nTime - pnode->nLastRecv);
             pnode->fDisconnect = true;