WIP [dbnode] Allow sharding of DB node client host queues for lower lock contention

[robskillington]

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What this PR does / why we need it:

Testing sharding host queues.

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

[robskillington]

Note: should consider using RDSCP per-CPU queue selection when resuming this.

[codecov[bot]]

Codecov Report

Merging #2291 (8422fe4) into master (527d5f5) will decrease coverage by 5.6%. The diff coverage is 72.1%.

@@           Coverage Diff            @@
##           master   #2291     +/-   ##
========================================
- Coverage    72.2%   66.7%   -5.6%     
========================================
  Files        1022    1016      -6     
  Lines       88793   88060    -733     
========================================
- Hits        64145   58739   -5406     
- Misses      20352   25093   +4741     
+ Partials     4296    4228     -68     

Flag	Coverage Δ
aggregator	80.4% <ø> (-1.7%)	:arrow_down:
cluster	67.5% <ø> (-17.9%)	:arrow_down:
collector	82.8% <ø> (ø)
dbnode	75.0% <72.1%> (-4.1%)	:arrow_down:
m3em	73.4% <ø> (-1.0%)	:arrow_down:
m3ninx	69.5% <ø> (-2.9%)	:arrow_down:
m3nsch	28.4% <ø> (-22.8%)	:arrow_down:
metrics	17.6% <ø> (ø)
msg	74.8% <ø> (-0.2%)	:arrow_down:
query	58.2% <ø> (-10.2%)	:arrow_down:
x	76.8% <ø> (-6.5%)	:arrow_down:

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