This is a low hanging fruit. Before this patch std::map::emplace() was
a ~50% of the whole recv_sys_t::parse() operation in by test.
After the fix it's only ~20%.

recv_sys_t::parse() recv_sys_t::pages is a collection of all pages
to recovery. Often, there are multiple changes for a single page.
Often, they go in a row and for such cases let's avoid
lookup in a std::map. cached_pages_it serves as a cache
of size 1.

recv_sys_t::add(): replace page_id argument with a std::map::iterator