Introduce the concept of a namespace

[big-andy-coates]

With reference to the PR that introduced TERMINATE ALL syntax.

We might look to introduce the concept of a namespace, (and a catalogue to track them), to which entities such as streams, tables, types, can be added.

We should support dropping a namespace as a unit. This will go some way to addressing issues like:

• Introduce a 'CASCADE' like keyword to enable efficient dropping of streams and tables. #2112
• DROP should have a flag to terminate dependent queries #1317
• DROP [STREAM|TABLE] should support termination of query started during creation. #2177

In the future, we may want to allow ACLs to be applied at the namespace level to allow/deny users access to them.

[big-andy-coates]

Closing as, while we may choose to add the concept of namespaces in the future, they are not needed to fix the issues with TERMINATE.

Instead, I'll be raising a KLIP proposing we drop INSERT INTO in favour of a SQL UNION, meaning all persistent queries are 1-2-1 with a data source. Then we make it so that dropping the source drops the persistent query, and then we can remove TERMINATE.