Snapshot TTL

Snapshots have a lifecycle that is independent from the table from which they are created. Although data in a table may be stored with TTL the data files containing them become frozen by the snapshot. Space consumed by expired cells will not be reclaimed by normal table housekeeping like compaction. While this is expected it can be inconvenient at scale. When many snapshots are under management and the data in various tables is expired by TTL some notion of optional TTL (and optional default TTL) for snapshots could be useful. It will help prevent the accumulation of junk files by automatically dropping the snapshot after the assigned TTL, making their data files eligible for cleaning. More comprehensive snapshot lifecycle management may be considered in the future but this one case is expected to be immediately useful given TTls on data are commonly applied for similar convenience.