read-cache: free threaded memory pool

read-cache: free threaded memory pool

In load_cache_entries_threaded(), each thread allocates its own memory
pool. This pool needs to be cleaned up while closing the threads down,
or it will be leaked.

This ce_mem_pool pointer could theoretically be converted to an inline
copy of the struct, but the use of a pointer helps with existing lazy-
initialization logic. Adjusting that behavior only to avoid this pointer
would be a much bigger change.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>