Infer ChangeAgeLimit from the TTL property of the CDC table

[piodul]

Each cdc-enabled table has a "cdc ttl" property which defines the TTL of the cdc log rows. Each row in the cdc log table disappears after that period of time - so there's no need to read changes which are older than that.

The library should take it into consideration:

• If reading of a stream starts from a very old saved position which is larger than the cdc ttl for the table, we should read starting from now() - cdc ttl instead.
• If there no saved progress and ChangeAgeLimit is larger than the cdc ttl, it makes more sense to start from now() - cdc ttl rather than now() - ChangeAgeLimit.

cc: @avelanarius