Make `Catalog` API case-insensitive.

[ryzhyk]

This will simplify integration with case-insensitive frontends like SQL. The only downside I can think of is that people won't be able to create streams whose names only differ in capitalization, but that doesn't sound like a good thing to do in any case.

@mbudiu-vmw

[mihaibudiu]

Maybe it should be configurable somehow.