multisite: intermittent failures in test_bucket_sync_disable_enable
The test writes some objects while bucket sync is disabled, and those objects sometimes don't sync after its reenabled. This is due to logic in RGWBucketShardIncrementalSyncCR that will continue with incremental sync if it sees both CLS_RGW_OP_SYNCSTOP and CLS_RGW_OP_RESYNC in the same bi listing. This isn't safe, though, because changes while sync is disabled don't add entries to the bilog. So the only way to guarantee that we don't miss any changes is to switch back to full sync instead.