[DBNode] - Add support for batching fetches across namespaces

[richardartoul]

No description provided.

[codecov]

Codecov Report

Merging #1987 into master will increase coverage by 0.1%.
The diff coverage is 92.9%.

@@            Coverage Diff            @@
##           master    #1987     +/-   ##
=========================================
+ Coverage    63.3%    63.5%   +0.1%     
=========================================
  Files        1111     1121     +10     
  Lines      105465   106163    +698     
=========================================
+ Hits        66841    67454    +613     
- Misses      34323    34392     +69     
- Partials     4301     4317     +16

Flag	Coverage Δ
#aggregator	79.7% <ø> (-0.1%)	⬇️
#cluster	56.4% <ø> (ø)	⬆️
#collector	63.7% <ø> (ø)	⬆️
#dbnode	65% <92.9%> (+0.1%)	⬆️
#m3em	59.6% <ø> (ø)	⬆️
#m3ninx	61.1% <ø> (+2.4%)	⬆️
#m3nsch	51.1% <ø> (ø)	⬆️
#metrics	17.7% <ø> (ø)	⬆️
#msg	74.7% <ø> (ø)	⬆️
#query	68.2% <ø> (ø)	⬆️
#x	75% <ø> (ø)	⬆️

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[justinjc]

LGTM