BUG/MINOR: cache: Remove incomplete entries from the cache when stream is closed

BUG/MINOR: cache: Remove incomplete entries from the cache when stream is closed

When a stream is interrupted by the client before the full answer is
stored in the cache, we end up with an incomplete entry in the cache
that cannot be overwritten until it "naturally" expires. In such a case,
we call the cache filter's cache_store_strm_deinit callback without ever
calling cache_store_http_end which means that the 'complete' flag is
never set on the concerned cache_entry.
This patch adds a check on the 'complete' flag in the strm_deinit
callback and removes the entry from the cache if it is incomplete.

A way to exhibit this bug is to try to get the same "big" response on
multiple clients at the same time thanks to h2load for instance, and to
interrupt the client side before the answer can be fully stored in the
cache.

This patch can be backported up to 2.4 but it will need some rework
starting with branch 2.8 because of the latest cache changes.