Make sure there is no parallel file write by `write_to_disk`

[saschanaz]

More details here: https://bugzilla.mozilla.org/show_bug.cgi?id=1807010#c2

rkv's safe mode protects the in-memory database contents with an RwLock (

rkv/src/backend/impl_safe/environment.rs

Lines 234 to 236 in c5a7594

	pub(crate) fn dbs_mut(&self) -> Result<RwLockWriteGuard<EnvironmentDbs>, ErrorImpl> {
	self.dbs.write().map_err(|_| ErrorImpl::EnvPoisonError)
	}

), but that lock is released in

rkv/src/backend/impl_safe/transaction.rs

Line 192 in c5a7594

drop(dbs);

, before flushing the contents to disk. So, even if (2) weren't a factor, multiple calls to EnvironmentImpl.write_to_disk() from different task queues could still race with each other.

I wonder we can mitigate this issue by adding a separate lock solely for this write_to_disk() and wait for the lock for each write.