[ACM] Only allow cache expiration values in full seconds.

[simitt]

fixes #2438

• changelog entry

backport to 7.x and 7.3.

[graphaelli]

looks good, even makes test confg fail if the value is bad. If we need this validation elsewhere we should pull it into a ucfg validator eg

apm-server/beater/config_helper.go

Line 30 in aa13cef

if err := ucfg.RegisterValidator("maxlen", func(v interface{}, param string) error {

[graphaelli]

Per discussion, we can update the integration test introduced in #2439 to check this.

[simitt]

I generally agree regarding the Unpack but we also use the same Cache struct for the sourcemapping cache and wouldn't want to introduce limitation there just now (might be a breaking change).