clientv3: v3Manager#saveDB should always close db file

[tedyu]

In v3Manager#saveDB, db file should always be closed upon return from the method.

Currently if db.Seek() call fails (around line 353), db is not closed.

[codecov-io]

Codecov Report

Merging #11838 into master will decrease coverage by 0.12%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master   #11838      +/-   ##
==========================================
- Coverage   66.03%   65.90%   -0.13%     
==========================================
  Files         403      403              
  Lines       36993    36999       +6     
==========================================
- Hits        24428    24386      -42     
- Misses      11054    11099      +45     
- Partials     1511     1514       +3     

Impacted Files	Coverage Δ
clientv3/snapshot/v3_snapshot.go	59.53% <50.00%> (-0.23%)	⬇️
client/members.go	51.61% <0.00%> (-33.88%)	⬇️
auth/range_perm_cache.go	54.28% <0.00%> (-14.29%)	⬇️
clientv3/naming/grpc.go	63.15% <0.00%> (-12.29%)	⬇️
clientv3/leasing/util.go	91.66% <0.00%> (-6.67%)	⬇️
auth/store.go	52.38% <0.00%> (-6.47%)	⬇️
pkg/adt/interval_tree.go	84.21% <0.00%> (-3.01%)	⬇️
proxy/grpcproxy/watch.go	88.75% <0.00%> (-2.96%)	⬇️
etcdserver/api/v3election/election.go	61.11% <0.00%> (-2.78%)	⬇️
proxy/httpproxy/director.go	65.85% <0.00%> (-2.44%)	⬇️
... and 24 more

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[tedyu]

@spzala @jingyih
Mind taking a look ?

[xiang90]

lgtm