gc the out-of-date and cold data

如果用户的写速度慢，compact不频繁，会导致TTL过期的数据长期得不到清理
尤其是最下层的数据，可能没人会跟它做compact
解决方案：
生成每个sst文件时，将体积占到一半的数据发生过期的时间记下，作为将来强制触发compact的时间点。对于最下层的文件，要支持单层compact

Start by tracing SST-file generation, TTL expiration handling, and compaction scheduling. Determine how to record when half of an SST's data expires and how the lowest level currently handles compaction. Done means expired cold data is eventually cleaned even with slow writes, including through supported single-level compaction at the lowest level.

Written by the indexing model from the issue text.