[improve][broker] Use immutable base cursor properties

[lhotari]

Motivation

• mutating the singleton base cursor properties accidentally would cause a hard-to-debug bug

Modifications

• use immutable maps for base cursor properties which are shared singletons

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

[eolivelli]

+1

[tisonkun]

LGTM.

Unfortunately the variable still have methods like put that only fail at the runtime.

[tisonkun]

LGTM.

Unfortunately the variable still have methods like put that only fail at the runtime.

[lhotari]

Unfortunately the variable still have methods like put that only fail at the runtime.

@tisonkun where do you see this?

[tisonkun]

@lhotari No. There is no issue in the current codebase.

I expect that when you view REPLICATED_SUBSCRIPTION_CURSOR_PROPERTIES at its use point, you still don't know it's immutable and can call put on the variable, which doesn't cause compile error but fail at runtime.