refactor(logstore): remove Entry::namemspace

[v0y4g3r]

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What's changed and what's your intention?

• Remove Entry::namespace method and related asscoiated type.
• Fix the estimated size logic to computing the encoded length of entries.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[tisonkun]

This is Entry::namespace()?

[tisonkun]

And create_namespace, delete_namespace, list_namespaces is unused also.

I suppose if we'd want to use the namespace encapsulation, we should use the Namespace type/trait, instead of pub ns: NamespaceImpl.

Otherwise, drop the whole namespace design and inline the concat namespace on demand.

[tisonkun]

LGTM. Although I'd prefer to declare our plan to the Namespace abstraction first and avoid directly access ns field if we'd hold this abstraction.

BTW, NoopLogStore is never used even in tests. We can remove it in a following PR since it is totally useless (the associated tests is noop actually).