[docdb] PITR-YSQL: Support with Tablespaces

[kripasreenivasan]

Jira Link: DB-636
Observed in builds: 2.8.0.0 b5, 2.9.1.0 b 132
With geo-partition being requirement, we need to ensure PITR support with tablespaces. Tablespace information is stored in the pg_tablespace system table which is global and not shared by databases. Currently, we don't restore the global objects of pg_catalog. For e.g. one scenario where this will have consequences:

1. Create a tablespace
2. Create a few tables that are a part of the tablespace created in (1)
3. Drop these tables
4. Drop the tablespace
5. Restore to a point after (2)

Since pg_tablespace isn't restored, the tablespace will continue to remain dropped even after restore leading to a failure in restoring the tables. There are several approaches we could take to mitigate this:

1. We could disable Drops on tablespaces - this would ensure that any table that we restore will have the corresponding tablespace information present always.
2. We could restore pg_tablespace as well - this can have unintended consequences because we might end up dropping some tablespaces containing tables of other databases.

Also, how does this interplay with the Alter Tablespace feature that we intend to support?