Add an API to recover kvdb from a backend DB file.

[adityadani]

• Added an API ReinitializeDB()
• Currently implemented only for etcd v3
• It is a part of a new Recovery interface
• It does not require an active kvdb client or connection to kvdb servers
  and is meant to be executed on the server itself

[codecov-io]

Codecov Report

Merging #82 into master will increase coverage by 0.28%.
The diff coverage is 85.18%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage   68.96%   69.24%   +0.28%     
==========================================
  Files           7        7              
  Lines        3415     3440      +25     
==========================================
+ Hits         2355     2382      +27     
+ Misses        826      824       -2     
  Partials      234      234

Impacted Files	Coverage Δ
etcd/v3/kv_etcd.go	67.76% <85.18%> (+0.82%)	⬆️
etcd/v2/kv_etcd.go	68.07% <0%> (-0.33%)	⬇️
consul/kv_consul.go	72.3% <0%> (+0.58%)	⬆️

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