[dbnode] Fix m3tsz encoding precision issue

[linasm]

What this PR does / why we need it:
There was avoidable precision loss in M3TSZ encoding. Accumulated multiplication was being done in a loop:

val = val * 10.0

Replaced this accumulation with multiplication by an precomputed lookup table (1, 10, 100, ...):

val := v * multipliers[int(mult)] * sign

This improves the precision of encoded datapoint values, as demonstrated by the new unit test.

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 #3872 (40bc7ff) into master (40bc7ff) will not change coverage.
The diff coverage is n/a.

❗ Current head 40bc7ff differs from pull request most recent head e3b288d. Consider uploading reports for the commit e3b288d to get more accurate results

@@          Coverage Diff           @@
##           master   #3872   +/-   ##
======================================
  Coverage    57.0%   57.0%           
======================================
  Files         552     552           
  Lines       63272   63272           
======================================
  Hits        36124   36124           
  Misses      23940   23940           
  Partials     3208    3208           

Flag	Coverage Δ
aggregator	64.3% <0.0%> (ø)
cluster	∅ <0.0%> (∅)
collector	58.4% <0.0%> (ø)
dbnode	60.6% <0.0%> (ø)
m3em	46.4% <0.0%> (ø)
metrics	19.7% <0.0%> (ø)
msg	74.2% <0.0%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 40bc7ff...e3b288d. Read the comment docs.

[robskillington]

LGTM