Re-add retired mempurge flag definitions for legacy-options-file temporary support...

Re-add retired mempurge flag definitions for legacy-options-file temporary support. (#8650)

Summary:
Current internal regression tests pass in an old option flag `experimental_allow_mempurge` to a more recently built db.
This flag was retired and removed in a recent PR (https://github.com/facebook/rocksdb/issues/8628), and therefore, the following error comes up : `Failed: Invalid argument: Could not find option: : experimental_allow_mempurge`.
In this PR, I reintroduce the two flags retired in https://github.com/facebook/rocksdb/issues/8628, `experimental_allow_mempurge` and `experimental_mempurge_policy` in `db_options.cc` and mark them both as `kDeprecated`.
This is a temporary fix to save us time to find a long term solution, which hopefully will consist in ignoring options prefixed with `experimental_` that are no longer recognized.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8650

Reviewed By: pdillinger

Differential Revision: D30257307

Pulled By: bjlemaire

fbshipit-source-id: 35303655fd2dd9789fd9e3c450e9d8009f3c1f54