WIP [dbnode] Concurrent stream metadata in single shard for higher peer stream throughput

[robskillington]

What this PR does / why we need it:

Work in progress.

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[bot]]

Codecov Report

Merging #2597 (25b6a8c) into master (225a0c4) will decrease coverage by 7.8%. Report is 1229 commits behind head on master. The diff coverage is 69.4%.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #2597      +/-   ##
=========================================
- Coverage    71.5%   63.7%    -7.8%     
=========================================
  Files        1083     816     -267     
  Lines       96391   78674   -17717     
=========================================
- Hits        68949   50185   -18764     
- Misses      22814   24606    +1792     
+ Partials     4628    3883     -745     

Flag	Coverage Δ
aggregator	63.5% <ø> (-12.5%)	:arrow_down:
cluster	84.6% <ø> (-0.5%)	:arrow_down:
collector	62.3% <ø> (-20.6%)	:arrow_down:
dbnode	67.2% <69.8%> (-12.2%)	:arrow_down:
m3em	63.7% <ø> (-10.7%)	:arrow_down:
m3ninx	65.4% <ø> (-6.9%)	:arrow_down:
m3nsch	78.0% <ø> (+26.8%)	:arrow_up:
metrics	17.2% <ø> (ø)
msg	75.3% <ø> (ø)
query	68.4% <20.0%> (+1.5%)	:arrow_up:
x	82.3% <75.0%> (+1.2%)	:arrow_up:

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