WritePrepared Txn: add write_committed option to dump_wal

WritePrepared Txn: add write_committed option to dump_wal

Summary:
Currently dump_wal cannot print the prepared records from the WAL that is generated by WRITE_PREPARED write policy since the default reaction of the handler is to return NotSupported if markers of WRITE_PREPARED are encountered. This patch enables the admin to pass --write_committed=false option, which will be accordingly passed to the handler. Note that DBFileDumperCommand and DBDumperCommand are still not updated by this patch but firstly they are not urgent and secondly we need to revise this approach later when we also add WRITE_UNPREPARED markers so I leave it for future work.

Tested by running it on a WAL generated by WRITE_PREPARED:
$ ./ldb dump_wal --walfile=/dev/shm/dbbench/000003.log  | grep BEGIN_PREARE | head -1
1,2,70,0,BEGIN_PREARE
$ ./ldb dump_wal --walfile=/dev/shm/dbbench/000003.log --write_committed=false | grep BEGIN_PREARE | head -1
1,2,70,0,BEGIN_PREARE PUT(0) : 0x30303031313330313938 PUT(0) : 0x30303032353732313935 END_PREPARE(0x74786E31313535383434323738303738363938313335312D30)
Closes https://github.com/facebook/rocksdb/pull/3682

Differential Revision: D7522090

Pulled By: maysamyabandeh

fbshipit-source-id: a0332207261c61e18b2f9dfbe9feecd9a1339aca