[dbnode] Log numShards on topology update

[linasm]

What this PR does / why we need it:
Just a minor logging change for better visibility.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:
NONE

Does this PR require updating code package or user-facing documentation?:
NONE

[codecov]

Codecov Report

Merging #3665 (ad5a4fd) into master (ad5a4fd) will not change coverage.
The diff coverage is n/a.

❗ Current head ad5a4fd differs from pull request most recent head f799ac8. Consider uploading reports for the commit f799ac8 to get more accurate results

@@          Coverage Diff           @@
##           master   #3665   +/-   ##
======================================
  Coverage    56.4%   56.4%           
======================================
  Files         551     551           
  Lines       62554   62554           
======================================
  Hits        35287   35287           
  Misses      24096   24096           
  Partials     3171    3171           

Flag	Coverage Δ
aggregator	57.1% <0.0%> (ø)
cluster	∅ <0.0%> (∅)
collector	58.4% <0.0%> (ø)
dbnode	60.9% <0.0%> (ø)
m3em	46.4% <0.0%> (ø)
metrics	19.8% <0.0%> (ø)
msg	74.4% <0.0%> (ø)

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