Make Configurable/Customizable options copyable (#8704)

Make Configurable/Customizable options copyable (#8704)

Summary:
The atomic variable "is_prepared_" was keeping Configurable objects from being copy-constructed.  Removed the atomic to allow copies.

Since the variable is only changed from false to true (and never back), there is no reason it had to be atomic.

Added tests that simple Configurable and Customizable objects can be put on the stack and copied.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8704

Reviewed By: anand1976

Differential Revision: D30530526

Pulled By: ltamasi

fbshipit-source-id: 4dd4439b3e5ad7fa396573d0b25d9fb709160576