Use one PermissibleBase for all Command Blocks - Fixes BUKKIT-4823

**Short summary**
Every Command Block creates a new instance of PermissibleBase. This can take several seconds depending on the amount of registered permissions and Command Blocks. In our case this caused our server to take more than 10 minutes to start.

**Link to the bug report**
https://bukkit.atlassian.net/browse/BUKKIT-4823

**What this does:**
This patch creates one single instance of PermissibleBase that will be used for every Command Block. Even though this is rather hacky it should not cause any issues as all Command Blocks should have the same permissions.