Fix use of make_unique in Arena::AllocateNewBlock (#11012)

Fix use of make_unique in Arena::AllocateNewBlock (#11012)

Summary:
The change to `make_unique<char[]>` in https://github.com/facebook/rocksdb/issues/10810 inadvertently started initializing data in Arena blocks, which could lead to increased memory use due to (at least on our implementation) force-mapping pages as a result. This change goes back to `new char[]` while keeping all the other good parts of https://github.com/facebook/rocksdb/issues/10810.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11012

Test Plan: unit test added (fails on Linux before fix)

Reviewed By: anand1976

Differential Revision: D41658893

Pulled By: pdillinger

fbshipit-source-id: 267b7dccfadaeeb1be767d43c602a6abb0e71cd0