Multiple tokens not combined correctly when computing service fallback

[DaoWen]

When computing fallback-period-secs and stale-timeout-mins for a service that has multiple source tokens, the current implementation just grabs these values from the last token in the list that sets them. The fallback logic should be more robust, e.g., taking the maximum value.

waiter/waiter/src/waiter/service_description.clj

Lines 1230 to 1234 in bc04a0b

	(let [{:strs [fallback-period-secs stale-timeout-mins]}
	(->> source-tokens
	(map #(some-> % :token token->token-parameters))
	(reduce merge {})
	(attach-token-defaults-fn))]

Alternatively, simply dropping support for combining tokens like this would also solve the problem. The biggest reason we haven't actually seen this bug in production is that no one uses this feature now that we support parameterized services.