Summary:
Previously `db_stress` attempted to configure direct I/O dynamically in `SetOptions()` which had multiple problems (ummm must've never been tested):

- It's a DB option so SetDBOptions should've been called instead
- It's not a dynamic option so even SetDBOptions would fail
- It required enabling SyncPoint to mask O_DIRECT since it had no way to detect whether the DB directory was in tmpfs or not. This required locking that consumed ~80% of db_stress CPU.

In this PR I delete the broken dynamic config and instead configure it statically, only enabling it if the DB directory truly supports O_DIRECT.
Closes https://github.com/facebook/rocksdb/pull/3939

Differential Revision: D8238120

Pulled By: ajkr

fbshipit-source-id: 60bb2deebe6c9b54a3f788079261715b4a229279