Without this change, when actually setting this conf opt, the tenant
would become Broken next time we load it.
Why?
The serde_toml representation that persist_tenant_conf would write out
would be a TOML inline table of `secs` and `nsecs`.
But our hand-rolled TenantConf parser expects a TOML string.
I checked that all other `Duration` values in TenantConfOpt
use the humantime serialization.
Issues like this would likely be systematically prevent by
https://github.com/neondatabase/neon/issues/3682