benchmarks: WAL thread should fdatasync in wal_hiccup
benchmarks: WAL thread should fdatasync in wal_hiccup

That's more representative of the Kudu WAL behavior.

Though the "slow WAL write" logging is triggered almost all the time now,
suggesting that maybe our notion of a "slow" WAL is wrong.

Change-Id: Iac7986f81835dc8c5b6b2e5bdfc8a257142ed56c
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5703
Tested-by: jenkins
Reviewed-by: Jean-Daniel Cryans <4bf4c125525b8623ac45dfd7774cbf531df19085@cloudera.com>

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