Synchronize access to @client_threads in LogStash::Outputs::Tcp

Logstash was consistently failing with the following error when a client connected to a TCP output:

```
NoMethodError: undefined method `write' for nil:NilClass
       register at /opt/logstash/lib/logstash/outputs/tcp.rb:92
           each at org/jruby/RubyArray.java:1613
       register at /opt/logstash/lib/logstash/outputs/tcp.rb:91
           call at org/jruby/RubyProc.java:271
         encode at /opt/logstash/lib/logstash/codecs/json.rb:45
        receive at /opt/logstash/lib/logstash/outputs/tcp.rb:143
         handle at /opt/logstash/lib/logstash/outputs/base.rb:86
     initialize at (eval):21
           call at org/jruby/RubyProc.java:271
         output at /opt/logstash/lib/logstash/pipeline.rb:266
   outputworker at /opt/logstash/lib/logstash/pipeline.rb:225
  start_outputs at /opt/logstash/lib/logstash/pipeline.rb:152
```

Logstash at the time was receiving a heavy stream of events, so this is likely the result of a race condition accessing `@client_threads` which is not thread safe, so this change synchronizes access to it across threads.