db: call Sync after deletion threshold

[darshanime]

Add periodic calls to (objstorage.Provider).Sync

Closes: #3025

[cockroach-teamcity]

This change is 

[RaduBerinde]

Thanks for the contribution!

Reviewable status: 0 of 3 files reviewed, 4 unresolved discussions (waiting on @darshanime)

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cleaner.go line 176 at r1 (raw file):

func (cm *cleanupManager) deleteObsoleteObjectAndSyncIfRequired(jobID int, fileNum base.DiskFileNum, fileSize uint64) {
	var deleteThresholdForSync uint64 = 64 * 1024 * 1024 // 64 MB

[nit] this should be a constant around the top of the file (next to jobsQueueDepth)

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cleaner.go line 188 at r1 (raw file):

	}

	if err := cm.objProvider.Sync(); err == nil {

We should log the error here. I think we should still reset deletedFileSizePendingSync even in the error case, it's unlikely that retrying after every object will help.

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cleaner.go line 265 at r1 (raw file):

}

func (cm *cleanupManager) deleteObsoleteObject(

This needs an explanation of the return value. Though I don't think it's worth it, the case of the object not existing is unlikely to happen in practice

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testdata/cleaner line 190 at r1 (raw file):

close: db1/000123.sst

create-bogus-file db1/000456.sst 100000000

Add a comment explaining that we're setting a large file size to verify that the cleaner issues a sync.