Initialize two members in PerfContext (#4859)

Summary:
as titled.
Currently it's possible to create a local object of type PerfContext since it's
part of public API. Then it's safe to initialize the two members to 0.
If PerfContext is created as thread-local object, then all members are
zero-initialized according to C++ standard.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4859

Differential Revision: D13614504

Pulled By: riversand963

fbshipit-source-id: 406ff548e105a074f379ad1054d56fece5f524a0