*: add flag to let etcd use the new boltdb freelistType feature

[WIZARD-CXY]

Use the new bboltdb feature(map freelist type) as a experimental feature in etcd to boost the performance when the data stored in etcd is huge(like 50GB).

[WIZARD-CXY]

@xiang90 ptal

[codecov-io]

Codecov Report

Merging #10444 into master will decrease coverage by 0.19%.
The diff coverage is 77.77%.

@@            Coverage Diff            @@
##           master   #10444     +/-   ##
=========================================
- Coverage   71.67%   71.48%   -0.2%     
=========================================
  Files         392      392             
  Lines       36495    36504      +9     
=========================================
- Hits        26158    26095     -63     
- Misses       8517     8590     +73     
+ Partials     1820     1819      -1

Impacted Files	Coverage Δ
etcdserver/config.go	79.51% <ø> (ø)	⬆️
etcdmain/config.go	82.48% <100%> (+0.08%)	⬆️
etcdserver/backend.go	52.83% <100%> (+0.9%)	⬆️
embed/etcd.go	71% <100%> (+0.11%)	⬆️
mvcc/backend/backend.go	74.15% <100%> (+0.09%)	⬆️
embed/config.go	56.37% <50%> (-0.08%)	⬇️
etcdctl/ctlv3/command/printer_simple.go	39.59% <0%> (-32.89%)	⬇️
etcdserver/api/v3rpc/util.go	43.47% <0%> (-26.09%)	⬇️
clientv3/namespace/watch.go	87.87% <0%> (-6.07%)	⬇️
pkg/adt/interval_tree.go	86.78% <0%> (-3.01%)	⬇️
... and 22 more

---

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[WIZARD-CXY]

@xiang90 ptal

[xiang90]

/cc @jpbetz

[WIZARD-CXY]

@xiang90 ptal

[xiang90]

lgtm after fixing the import line.

[jingyih]

Looks good. Just few nits.

[WIZARD-CXY]

@xiang90 @jingyih changed according to your suggestions. ptal

[xiang90]

lgtm