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

[yyin-sc]

we should return error here, no?

[robskillington]

We check this at the end, can't return error here unfortunately since it's an async goroutine.

[codecov]

Codecov Report

❌ Patch coverage is 69.42675% with 96 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.7%. Comparing base (225a0c4) to head (25b6a8c).
⚠️ Report is 1302 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (225a0c4) and HEAD (25b6a8c). Click for more details.

HEAD has 60 uploads less than BASE

Flag	BASE (225a0c4)	HEAD (25b6a8c)
collector	16	6
aggregator	15	6
m3em	12	6
cluster	13	6
msg	5	4
dbnode	13	6
query	8	2
m3ninx	7	2
m3nsch	7	2
x	6	2

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%)	⬇️
cluster	84.6% <ø> (-0.5%)	⬇️
collector	62.3% <ø> (-20.6%)	⬇️
dbnode	67.2% <69.8%> (-12.2%)	⬇️
m3em	63.7% <ø> (-10.7%)	⬇️
m3ninx	65.4% <ø> (-6.9%)	⬇️
m3nsch	78.0% <ø> (+26.8%)	⬆️
metrics	17.2% <ø> (ø)
msg	75.3% <ø> (ø)
query	68.4% <20.0%> (+1.5%)	⬆️
x	82.3% <75.0%> (+1.2%)	⬆️

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