[docdb] PITR: Add persistent tracking for DDL statements

[bmatican]

Jira Link: DB-1829
To start with, this can be a new type of sys_catalog entry, written to from every DDL entry point. We can use a simple time based query interface, to be able to see the last N entries / last M hours of entries, via yb-admin.

This will help for PITR, as we can then be able to rollback to right before executing a particular DDL statement:

• For YCQL, the timestamp of the actual DDL record, written from CatalogManager, might be good enough.
• For YSQL, we might want to integrate better with the PG side of tracking, to record the start and end of a DDL transaction.

[bmatican]

After the initial proof of concept, we should augment this with

• enhance to track more types of DDLs, eg: create table is not tracked atm
• client side filtering, eg: which record types, or which namespaces / tables are relevant, what time window
• pagination, eg: LIMIT will be super important, to not overload the server on a request OR the client based on the RPC size, but at that point, we might as well have limit and offset
• some debug interface, aside from just a yb-admin command, eg: either a master UI page, or RPCs that YW can call and build a UI on top of

cc @sanketkedia

[sanketkedia]

I am taking point on this. cc @spolitov to avoid duplication.

[sanketkedia]