Add cxy_ctl_sup to properly own ets tables

[jaynel]

Calling cxy_ctl:init or cxy_ctl:add_task_types causes a new ets_table to be created. The caller will be the owner of the ets table. If a client application calls these library functions from the wrong process, the ets table might get deleted accidentally when the caller's process terminates.

To make the library safer to use, a supervisor should be provided that starts a gen_server dedicated to owning the ets tables. The API for this gen_server should support creating and adjusting the limits, but leave it up to the client to directly call the cxy_ctl functions for executing tasks.