Better rank caching strategy.

CHANGELOG.md

build/Dockerfile

internal/skiplist/README.md

internal/skiplist/element.go

internal/skiplist/skiplist.go

internal/skiplist/skiplist_test.go

server/api_test.go

server/core_storage_test.go

server/core_wallet_test.go

server/leaderboard_rank_cache.go

server/leaderboard_rank_cache_test.go

server/runtime_go_logger_test.go

server/runtime_test.go