MEDIUM: shctx: Remove 'hot' list from shared_context

MEDIUM: shctx: Remove 'hot' list from shared_context

The "hot" list stored in a shared_context was used to keep a reference
to shared blocks that were currently being used and were thus removed
from the available list (so that they don't get reused for another cache
response). This 'hot' list does not ever need to be shared across
threads since every one of them only works on their current row.

The main need behind this 'hot' list was to detach the corresponding
blocks from the 'avail' list and to have a known list root when calling
list_for_each_entry_from in shctx_row_data_append (for instance).

Since we actually never need to iterate over all members of the 'hot'
list, we can remove it and replace the inc_hot/dec_hot logic by a
detach/reattach one.