TODO: obey cmdq queue length limit

[cederom]

As reported by @MemphisCZ in https://github.com/cederom/LibSWD/issues/6:

...and one more question about memory consumption. Where are the points in source code for cmdq memory release. I found function libswd_cmdq_free_head, but it is nowhere called, so after few rutins it eats all my 64k embedded memory :-D How can I find the right moment for cmdq release?