server: refuse a DELETE of an unknown name and decide downloads in load()
A DELETE of a name that is neither a model nor a recorded download
went to common_download_remove, and without a tag it wiped the whole
repo cache, including the files of a loaded quant or of a download on
its way. Such a name is now not found, as it was before.
A POST that downloads over an existing entry checked its state outside
the lock, so a load of the same model in between got its instance
killed. The checks now run in load(), under the lock, right before the
spawn. The fake load time of the tests applies to model loads only,
since a download worker loads none.