[MCL-6124] Concurrent logging causes malformed log lines

The bug

Two messages which are logged concurrently, for example one by the client and one by the server, cause malformed log lines.

                    Using VBOs: Yes
                    Is Modded: Probably not. Jar signature remains and client brand is untouched.
                    Type: Client (map_client.txt)
                    Resource Packs: 
                    Current Language: English (US)
                    Profiler Position: N/A (disabled)
                    CPU: 6x AMD Phenom(tm) II X6 1055T Processor
                  <log4j:Event logger="kz" timestamp="1486995793740" level="INFO" thread="File IO Thread">
                    <log4j:Message><![CDATA[[STDERR]: 	at bdr.run(SourceFile:30)]]></log4j:Message>
                  </log4j:Event>
16:23:13 monitor expected <
16:23:13 game    #@!@# Game crashed! Crash report saved to: #@!@# C:\Users\###
                  <log4j:Event logger="kz" timestamp="1486995793742" level="INFO" thread="File IO Thread">
                    <log4j:Message><![CDATA[[STDERR]: 	at java.lang.Thread.run(Thread.java:745)]]></log4j:Message>
                  </log4j:Event>
16:23:13 monitor expected <

Additionally affected versions

2.0.1034-stage (Windows)
2.1.1462 (Windows)

How to reproduce

Create a world which takes some time to save, for example an amplified world
Start pressing F3 + C to trigger a crash
Right before Minecraft crashes(should happen after 10 seconds of pressing the keys) press ESC to have Minecraft save the world

You might have to repeat this multiple times to reproduce it.