Don't put entire actor registry in debug string since it's too long

[ericl]

No description provided.

[AmplabJenkins]

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[ericl]

Ping

[robertnishihara]

@ericl Compilation error

In file included from /Users/travis/build/ray-project/ray/src/ray/raylet/actor_registration.cc:1:
/Users/travis/build/ray-project/ray/src/ray/raylet/actor_registration.h:107:8: error: private field 'alive_' is not used [-Werror,-Wunused-private-field]
  bool alive_;
       ^
1 error generated.
make[2]: *** [src/ray/CMakeFiles/ray_objlib.dir/raylet/actor_registration.cc.o] Error 1
make[1]: *** [src/ray/CMakeFiles/ray_objlib.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
3 warnings generated.

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