[ledger storage] DbLedgerStorage should do periodical flush

[sijie]

Descriptions of the changes in this PR:

Motivation

DbLedgerStorage doesn't drive checkpoint itself. so currently DbLedgerStorage flush only
happens either when write-cache is full or entry log file rotated. The correctness is still
maintained. However the behavior is different from original yahoo behavior.

Changes

Revert the behavior back to original periodical flush

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[merlimat]

Change LGTM. Maybe we could also have a method in the LedgerStorage interface that each implementation can use to declare if it wants to have the sync thread to be scheduled or not.

[sijie]

@merlimat

Maybe we could also have a method in the LedgerStorage interface that each implementation can use to declare if it wants to have the sync thread to be scheduled or not.

was thinking about doing that in a subsequent change. because this change is going to all 3 branches, so better to keep the change simple, so I don't encounter any issue when cherry-pick it to branches.

[merlimat]

was thinking about doing that in a subsequent change. because this change is going to all 3 branches, so better to keep the change simple, so I don't encounter any issue when cherry-pick it to branches.

Sure, let's do that in master only.

[sijie]

run integration tests