Cleanup node management logic

[fcrisciani]

Created method to handle the node state change with cleanup operation
associated.
Realign testing client with the new diagnostic interface

Signed-off-by: Flavio Crisciani [email protected]
(cherry picked from commit b499aa9)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (bump_17.12@9bca9a4). Click here to learn what that means.
The diff coverage is 81.7%.

@@              Coverage Diff              @@
##             bump_17.12    #2036   +/-   ##
=============================================
  Coverage              ?   40.18%           
=============================================
  Files                 ?      138           
  Lines                 ?    22135           
  Branches              ?        0           
=============================================
  Hits                  ?     8895           
  Misses                ?    11935           
  Partials              ?     1305

Impacted Files	Coverage Δ
networkdb/networkdbdiagnose.go	0% <0%> (ø)
networkdb/event_delegate.go	68.57% <50%> (ø)
networkdb/delegate.go	75% <73.33%> (ø)
networkdb/nodemgmt.go	92.59% <92.59%> (ø)

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