No need to hold server write lock since sendq has its own.

I noticed some contention when I was investigating a catchup bug on the server write lock.
Medium term we could have a separate lock, longer term formal client support in the server will alleviate.

Signed-off-by: Derek Collison <derek@nats.io>

server/events.go

@@ -481,9 +481,9 @@ func (s *Server) sendInternalAccountMsg(a *Account, subject string, msg interfac
 
 // Used to send an internal message with an optional reply to an arbitrary account.
 func (s *Server) sendInternalAccountMsgWithReply(a *Account, subject, reply string, hdr map[string]string, msg interface{}, echo bool) error {
-	s.mu.Lock()
+	s.mu.RLock()
 	if s.sys == nil || s.sys.sendq == nil {
-		s.mu.Unlock()
+		s.mu.RUnlock()
 		return ErrNoSysAccount
 	}
 	c := s.sys.client
@@ -494,16 +494,16 @@ func (s *Server) sendInternalAccountMsgWithReply(a *Account, subject, reply stri
 		a.mu.Unlock()
 	}
 	s.sys.sendq.push(newPubMsg(c, subject, reply, nil, hdr, msg, noCompression, echo, false))
-	s.mu.Unlock()
+	s.mu.RUnlock()
 	return nil
 }
 
 // This will queue up a message to be sent.
 // Lock should not be held.
 func (s *Server) sendInternalMsgLocked(subj, rply string, si *ServerInfo, msg interface{}) {
-	s.mu.Lock()
+	s.mu.RLock()
 	s.sendInternalMsg(subj, rply, si, msg)
-	s.mu.Unlock()
+	s.mu.RUnlock()
 }
 
 // This will queue up a message to be sent.
@@ -517,13 +517,13 @@ func (s *Server) sendInternalMsg(subj, rply string, si *ServerInfo, msg interfac
 
 // Will send an api response.
 func (s *Server) sendInternalResponse(subj string, response *ServerAPIResponse) {
-	s.mu.Lock()
+	s.mu.RLock()
 	if s.sys == nil || s.sys.sendq == nil {
-		s.mu.Unlock()
+		s.mu.RUnlock()
 		return
 	}
 	s.sys.sendq.push(newPubMsg(nil, subj, _EMPTY_, response.Server, nil, response, response.compress, false, false))
-	s.mu.Unlock()
+	s.mu.RUnlock()
 }
 
 // Used to send internal messages from other system clients to avoid no echo issues.
@@ -535,13 +535,13 @@ func (c *client) sendInternalMsg(subj, rply string, si *ServerInfo, msg interfac
 	if s == nil {
 		return
 	}
-	s.mu.Lock()
+	s.mu.RLock()
 	if s.sys == nil || s.sys.sendq == nil {
-		s.mu.Unlock()
+		s.mu.RUnlock()
 		return
 	}
 	s.sys.sendq.push(newPubMsg(c, subj, rply, si, nil, msg, noCompression, false, false))
-	s.mu.Unlock()
+	s.mu.RUnlock()
 }
 
 // Locked version of checking if events system running. Also checks server.