[MC-68374] ConcurrentModificationException when performing save-all via rcon

Issuing save-all on a server via RCON sometimes causes a (non-fatal) exception to be logged:

[10:48:57] [RCON Client #2/ERROR]: Couldn't process command: 'save-all'
java.util.ConcurrentModificationException
        at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:819) ~[?:1.7.0_09]
        at java.util.ArrayList$Itr.next(ArrayList.java:791) ~[?:1.7.0_09]
        at qt.a(SourceFile:771) ~[minecraft_server.14w34c.jar:?]
        at cg.a(SourceFile:37) ~[minecraft_server.14w34c.jar:?]
        at ab.a(SourceFile:80) [minecraft_server.14w34c.jar:?]
        at ab.a(SourceFile:63) [minecraft_server.14w34c.jar:?]
        at net.minecraft.server.MinecraftServer.g(SourceFile:789) [minecraft_server.14w34c.jar:?]
        at th.run(SourceFile:80) [minecraft_server.14w34c.jar:?]
        at java.lang.Thread.run(Thread.java:722) [?:1.7.0_09]