Relax the MAX_TTL restriction

[pjenvey]

We currently limit ttl to a max of 31_536_000, matching the original restriction in Python syncstorage.

It "temporarily" lifted this limit to work around a bug (see Issue 977397 / server-syncstorage#80b57eab but never restored it. Since then the forms engine began defaulting to a higher value.

Let's bump the value to at the very least forms' 3 years or some other agreed upon number.

The Sync 1.5 API states a max of 9 digits but that's significantly large (potentially > 31 years). If possible we should update the API doc with the a new max.

[pjenvey]

Mark brought up a good point that the default TTL is a "forever" value of 2_100_000_000 (=~ 66 years) so maybe =~ 31 years doesn't matter much.

[tublitzed]

For the record the diff where we changed the FORMS_TTL is here and happened 5 years ago.