Fix ets_lru configuration in chttpd application

[iilyak]

Overview

The code was incorrect in a sense that it was using is_integer guard,
while config:get cannot return integer.

Testing recommendations

1. Update default.ini to set max_lifetime to 10000
2. Restart chttpd application
3. Try manually insert keys into lru_cache and query them. They all should be removed after 10 seconds

lists:foreach(fun(Idx) -> ets_lru:insert(chttpd_auth_cache_lru, Idx, [1,2,3]) end, lists:seq(1, 1000)).
lists:filter(fun(Idx) -> ets_lru:lookup(chttpd_auth_cache_lru, Idx) /= not_found end, lists:seq(1, 1000)).

4. Try negative value for the setting check logs to make sure it complains
5. Try putting "some_string" as value check logs to make sure it complains

Related Issues or Pull Requests

N/A

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;

[rnewson]

the simpler version is to use config:get_integer

[iilyak]

@rnewson Updated to use config:get_integer