Summary:
both options `ttl` and `periodic_compaction_seconds` have the same meaning for FIFO compaction, which is redundant and can be confusing to use. For example, setting TTL to 0 does not disable TTL: user needs to also set periodic_compaction_seconds to 0. Another example is that dynamically setting `periodic_compaction_seconds` (surprisingly) has no effect on TTL compaction. This is because FIFO compaction picker internally only looks at value of `ttl`. The value of `ttl` is in `SanitizeOptions()` which take into account the value of `periodic_compaction_seconds`, but dynamically setting an option does not invoke this method.

This PR clarifies the usage of both options for FIFO compaction: only `ttl` should be used, `periodic_compaction_seconds` will not have any effect on FIFO compaction.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11550

Test Plan:
- updated existing unit test `DBOptionsTest.SanitizeFIFOPeriodicCompaction`
- checked existing values of both options in feature matrix: https://fburl.com/daiquery/xxd0gs9w. All current uses cases either have `periodic_compaction_seconds = 0` or have `periodic_compaction_seconds > ttl`, so should not cause change of behavior.

Reviewed By: ajkr

Differential Revision: D46902959

Pulled By: cbi42

fbshipit-source-id: a9ede235b276783b4906aaec443551fa62ceff4c