[dbnode] Remove invariant possibility when reading from enqueue channel

[robskillington]

What this PR does / why we need it:

Fixes #

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 #1980 into master will increase coverage by <.1%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #1980     +/-   ##
=========================================
+ Coverage    63.4%    63.4%   +<.1%     
=========================================
  Files        1119     1119             
  Lines      106004   106004             
=========================================
+ Hits        67278    67297     +19     
+ Misses      34414    34400     -14     
+ Partials     4312     4307      -5

Flag	Coverage Δ
#aggregator	79.7% <0%> (-0.1%)	⬇️
#cluster	56.3% <0%> (ø)	⬆️
#collector	63.7% <0%> (ø)	⬆️
#dbnode	64.9% <0%> (ø)	⬆️
#m3em	59.6% <0%> (ø)	⬆️
#m3ninx	61.1% <0%> (ø)	⬆️
#m3nsch	51.1% <0%> (ø)	⬆️
#metrics	17.7% <0%> (ø)	⬆️
#msg	74.7% <0%> (ø)	⬆️
#query	68.2% <0%> (ø)	⬆️
#x	75% <0%> (ø)	⬆️

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[richardartoul]

LGTM