storm-tape is slow

The current storm-tape is not fast enough to satisfy the expected usage by LHC experiments, see for example https://ggus.eu/index.php?mode=ticket_info&ticket_id=164961.

The purpose of this issue is to track a few changes to address the problem, such as:

• correct use of the db indexes
• review the implementation of some functionality, e.g. the in_progress request
• the possibility to run storm-tape in "mirror mode", see https://nginx.org/en/docs/http/ngx_http_mirror_module.html

Note that storm-tape is single-threaded; before moving to multi-threading let's try to make it as fast as possible.