This commit adds a new Cache option, via the
`tsm1.CacheConfig.SnapshotAgeDuration` field, which controls the maximum
age the cache can reach before it is snapshotted to a TSM file.
The default value for this option is `0`, which means that the cache
will never be snapshotted based only on age. Setting this value to, for
example, 10 seconds, would result in the cache snapshotting every 10
seconds.
Snapshotting the cache more frequently can provide better durability
guarantees in some circumstances, though more, smaller TSM files will
lead to more work needed to compact them down to larger, more dense
files.
When using InfluxDB with a WAL there isn't really a strong reason to
alter `tsm1.CacheConfig.SnapshotAgeDuration` from `0`.