(LMDB) Correct use of mdb_env_sync

["Pierre Chapuis" <catwell@archlinux.us>]

I am having a hard time understanding how exactly mdb_env_sync should
be used. Suppose I have an environment with MDB_NOSYNC and I do not use
MDB_WRITEMAP. I still want to sync at some points, which is why
mdb_env_sync exists. But I wonder:

- How does it interact with transactions? Is it safe (or necessary) to
  call it outside of a transaction? During a read-only transaction?
  During a write transaction?

- What about thread safety? Can it be called concurrently with writes?
  Can it be called concurrently with another mdb_env_sync in another
  thread?

-- 
Pierre Chapuis