Unlock during trying to get ZK-lock

This PR is the patch for #618.

I tested this patch in below condition:

* Number of ``jubaclassifier`` process: 4
* Number of RPC thread: 4
* MIX interval count: 1000
* Number of Client: 96
* Ratio of ``train`` and ``classifier``: 10 : 90

Result is followings:

               | QPS      | MAX CPU Usage | MIX count per 10 min
-------------- | -------- | --------- | ---------
w/ this patch  | 23836.75 | 79%       | 325
wo/ this patch | 12806.05 | 56%       | 193