Summary:
Do not bother comparing the version of the local super version handle with the global one.

An inequality comparison result indicates nothing but a spurious obsoleteness. It only happens when the writer has increased the `ColumnFamilyData::super_version_number_`(https://github.com/facebook/rocksdb/blob/5fc57eec2b44337289f25c1b5687beb54ad709a2/db/column_family.cc#L1309) but has not yet called `ResetThreadLocalSuperVersions()`(https://github.com/facebook/rocksdb/blob/5fc57eec2b44337289f25c1b5687beb54ad709a2/db/column_family.cc#L1328) at the time when a reader reads the local handle(`void* ptr = local_sv_->Swap(SuperVersion::kSVInUse);`). In other words, the existence of a local handle is a sufficent evidence of its fressness.

With this PR, we save one or even two atomic instructions when getting a handle of super version.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11452

Reviewed By: ajkr

Differential Revision: D46059317

Pulled By: cbi42

fbshipit-source-id: 68b4b1ca8a9929a4aa470105c37a09e0625b014d