Ignore off-by-one differences when optimizing sharded placement

[MantasMiksys]

What this PR does / why we need it:

Previously, sharded placement optimization (optimize()) which is performed during instance removal, replica addition and instance replacement (if AllowPartialReplace config is enabled), would cause unnecessary shard movement.
For example, if there were 3 instances A, B, C with 2, 2, 1 shards respectively, then replacement of node A with a new instance D would also trigger a movement of 1 shard from B to C.

To fix this, the way underloaded instance is determined was changed. In addition to checking if any instance has less than its target load of shards, a check for any instance having more than its target load of shards was added. The second check will fail and no optimization will be performed in scenarios like the one described above.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

Does this PR require updating code package or user-facing documentation?:

[codecov]

Codecov Report

Merging #3532 (d051ea4) into master (d051ea4) will not change coverage.
The diff coverage is n/a.

❗ Current head d051ea4 differs from pull request most recent head 79dd9d0. Consider uploading reports for the commit 79dd9d0 to get more accurate results

@@          Coverage Diff           @@
##           master   #3532   +/-   ##
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  Coverage    55.9%   55.9%           
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  Files         549     549           
  Lines       61434   61434           
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  Hits        34366   34366           
  Misses      23966   23966           
  Partials     3102    3102           

Flag	Coverage Δ
aggregator	57.1% <0.0%> (ø)
cluster	∅ <0.0%> (∅)
collector	54.3% <0.0%> (ø)
dbnode	60.3% <0.0%> (ø)
m3em	46.4% <0.0%> (ø)
metrics	19.8% <0.0%> (ø)
msg	74.6% <0.0%> (ø)

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