Custom Partition Key in PeerDB UI

[iskakaushik]

Currently, though the backend supports partitioning on a column of type int or datetime, we only expose CTID-based partitioning. This is a feature request to support selecting a custom column as the partition key. In the UI, we can show a drop-down for each table, default the value to ctid, and allow the user to select a custom column per table as they wish, only allowing valid types though.

Another important aspect of this is: if a user selects a custom, non-CTID partition key for all the tables as part of this mirror, there is no reason to maintain a long snapshot connection. We create the slot, and then we don't need a long-living connection; we can let the connection close right away since the changes are captured in the slot anyway. This is especially useful in cases where there are many transactions preventing vacuums and causing database issues.