Multi Table Transactions

h3. Strawman idea: 
* Introduce a notion of a "database" into Hudi's core (think of it, analogous to a database server), with its own timeline. 
* We introduce a parent-child relationship between the table's timeline and the database timeline i.e an action is complete only if its completed in both timelines (similar to data <=> metadata table sync today; although we can't reuse that)
* A multi table transaction will first create the action on the database timeline, then perform actions on individual tables, then finally complete it on the database timeline. 

h3. Open items: 
* Need to formalize the design with considerations around isolation levels, nested queries, self joins, avoid phantom reads. 
* Need to layout how we can deliver this via API and SQL (Spark for now) 
* How this interplays with multi-writer scenarios and async table services. 

## JIRA info

- Link: https://issues.apache.org/jira/browse/HUDI-6709
- Type: Epic
- Fix version(s):
- 1.1.0