Summary:
A user who extended `Logger` recently pointed out it is unusual to
require they implement the two-argument `Logv()` overload when they've
already implemented the three-argument `Logv()` overload. I agree with
that and think we can fix it by only calling the two-argument overload
from the default implementation of the three-argument overload. Then
when the three-argument overload is overridden, RocksDB would not
rely on the two-argument overload. Only `Logger::LogHeader()` needed
adjustment to achieve this.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7605

Reviewed By: riversand963

Differential Revision: D24584749

Pulled By: ajkr

fbshipit-source-id: 9aabe040ac761c4c0dbebc4be046967403ecaf21