*: add flags to setup backend related config

[xiang90]

The default backend batch interval is 100ms, which is too long for a fast local ssd. It means that a SSD will be almost idle for 100ms for another fdatasync to happen.

Making backend related configuration to be configurable through flags.

[xiang90]

@gyuho

We probably should change the default value from 100ms,10000 to 10ms, 1000 regardless. The current value is too conservative and might hurt latency.

It would be great if you can spend some time to re-do some benchmark with the new value I suggested before changing them.

[gyuho]

@xiang90 Yeah, I will do it before the v3.4 release!

[gyuho]

lgtm
/cc @jpbetz

[codecov-io]

Codecov Report

Merging #10283 into master will decrease coverage by 0.28%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master   #10283      +/-   ##
==========================================
- Coverage   71.89%    71.6%   -0.29%     
==========================================
  Files         391      391              
  Lines       36374    36386      +12     
==========================================
- Hits        26150    26055      -95     
- Misses       8420     8521     +101     
- Partials     1804     1810       +6

Impacted Files	Coverage Δ
etcdserver/config.go	79.51% <ø> (ø)	⬆️
embed/config.go	56.44% <ø> (ø)	⬆️
etcdserver/backend.go	51.92% <0%> (-9.45%)	⬇️
etcdmain/config.go	83.11% <100%> (+0.15%)	⬆️
embed/etcd.go	70.03% <100%> (+0.11%)	⬆️
proxy/grpcproxy/register.go	69.44% <0%> (-11.12%)	⬇️
pkg/netutil/netutil.go	63.11% <0%> (-7.38%)	⬇️
pkg/tlsutil/tlsutil.go	86.2% <0%> (-6.9%)	⬇️
pkg/fileutil/purge.go	65.9% <0%> (-6.82%)	⬇️
clientv3/leasing/util.go	91.66% <0%> (-6.67%)	⬇️
... and 28 more

---

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[jpbetz]

lgtm

+1 to decreasing the defaults for 3.4