Handle other hash_add_unique() errors on cache_get_and_ref_entry()

On unknown errors from hash_add_unique(), mark the item as a to-die-soon,
floating, and with one reference only.
Get rid of _create_temporary_cache_entry()

If the cache would block on read, just yield from the coroutine to try
again. Getting rid of one less common way of serving a file means less
tested code is not around to be a problem later on.

Next step in cleaning file serving is to cache file not found errors, since
cache misses are expensive.
Simplify serve_files_handle_cb().

The logic in this function has been superseded by whatever the cache
performs these days.

Also fix a string comparison in _create_cache_entry(), which I have no idea
how it worked before.
Plug memory leak in _create_cache_entry()
Remove priv parameter from serve callbacks

The value from priv can be deduced from the request pointer.
Remove unneeded #include directives from file serving stuff