Change std::vector<map_cache_t> to std::map<uint64_t, map_cache_t>

In api lib, currently std::vector is used to store the map cache. It will be more efficient to change it to std::map.

Start by locating the API library code that stores the map cache as std::vector and trace all accesses to that cache. Check how the uint64_t key should map to each map_cache_t, then update the affected uses and run the relevant API library tests to confirm cache behavior remains correct.

Written by the indexing model from the issue text.