Remove the import thread from the workers and driver.

[robertnishihara]

Right now the import thread fetches remote function definitions, actor definitions, custom class definitions (needed for custom serialization callbacks). It should be possible to get rid of the extra thread and to simply fetch these values from Redis when they are needed (though that could add latency the first time a remote function is invoked on a worker).

This would also address an issue brought up in #824.

[imzhenyu]

Thx, @robertnishihara . This actually reminds me another problem - do we have GC capability in python for remote functions? I.e., whether we support long running services for people continuously submitting new ray jobs (we did that on the java side and we want to co-deploy the python as well, as a shared ray job cluster)?

[robertnishihara]

Currently there is no GC for remote functions in Redis (or in the Python worker processes), but it would not too challenging to associate each remote function with the ID of the driver that defined it and to remove it when the driver exits.

I just created issue #958 to track this.