storage: flip the default for kv.raft.log.sync

[petermattis]

1. Quantify the slow down for enable syncing of the Raft log.
2. Flip the default from false to true.

[petermattis]

kv.raft.log.sync.enabled=false:

95% reads:
_elapsed___errors____________ops___ops/sec(cum)__p95(ms)__p99(ms)_pMax(ms)
  600.0s        0       28084309        46804.9     14.7     39.8    218.1

0% reads:
_elapsed___errors____________ops___ops/sec(cum)__p95(ms)__p99(ms)_pMax(ms)
  600.0s        0       11005957        18342.8     39.8     71.3    385.9

kv.raft.log.sync.enabled=true:

95% reads:
_elapsed___errors____________ops___ops/sec(cum)__p95(ms)__p99(ms)_pMax(ms)
  600.0s        0       26518608        44194.0     17.8     41.9    218.1

0% reads:
_elapsed___errors____________ops___ops/sec(cum)__p95(ms)__p99(ms)_pMax(ms)
  600.0s        0        6761876        11269.5     44.0     79.7    604.0

There is a 40% performance hit on the write-only workload. Interestingly, idle CPU was decently high when kv.raft.log.sync.enabled=true on that workload.