leaderboard rank cache improvements (#1050)

CHANGELOG.md

internal/skiplist/element.go

internal/skiplist/skiplist.go

internal/skiplist/skiplist_test.go

server/api_account.go

server/api_leaderboard_test.go

server/api_test.go

server/config.go

server/console_account.go

server/core_account.go

server/core_leaderboard.go

server/core_tournament.go

server/db.go

server/leaderboard_rank_cache.go

server/leaderboard_rank_cache_test.go

server/runtime_go_nakama.go

server/runtime_javascript_nakama.go

server/runtime_lua_nakama.go

server/runtime_test.go

vendor/github.com/stretchr/testify/require/doc.go