Support marking snapshots for write-conflict checking
Summary: D50475 enables using SST files for transaction write-conflict checking. In order for this to work, we need to make sure not to compact out SingleDeletes when there is an earlier transaction snapshot(D50295). If there is a long-held snapshot, this could reduce the benefit of the SingleDelete optimization. This diff allows Transactions to mark snapshots as being used for write-conflict checking. Then, during compaction, we will be able to optimize SingleDeletes better in the future. This diff adds a flag to SnapshotImpl which is used by Transactions. This diff also passes the earliest write-conflict snapshot's sequence number to CompactionIterator. This diff does not actually change Compaction (after this diff is pushed, D50295 will be able to use this information). Test Plan: no behavior change, ran existing tests Reviewers: rven, kradhakrishnan, yhchiang, IslamAbdelRahman, sdong Reviewed By: sdong Subscribers: dhruba, leveldb Differential Revision: https://reviews.facebook.net/D51183
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