WIP [dbnode] Buffer encoded commit log data and finalize batch resources async

[robskillington]

What this PR does / why we need it:

Allows pending IO to queue up while encode loop for the commit log can continue working.

Also reserves an entire gomaxproc for encode loop to avoid scheduling latency/being pre-empted while in the middle of critical encode loop.

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 #2395 (c4a34a3) into master (b239df4) will decrease coverage by 0.1%. Report is 1357 commits behind head on master. The diff coverage is 85.7%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2395     +/-   ##
========================================
- Coverage    71.7%   71.6%   -0.1%     
========================================
  Files        1052    1051      -1     
  Lines       93115   92958    -157     
========================================
- Hits        66797   66613    -184     
- Misses      21874   21894     +20     
- Partials     4444    4451      +7     

Flag	Coverage Δ
aggregator	76.4% <ø> (-0.1%)	:arrow_down:
cluster	84.8% <ø> (ø)
collector	82.8% <ø> (ø)
dbnode	79.1% <85.7%> (-0.1%)	:arrow_down:
m3em	74.4% <ø> (ø)
m3ninx	73.1% <ø> (+<0.1%)	:arrow_up:
m3nsch	51.1% <ø> (ø)
metrics	17.3% <ø> (ø)
msg	75.2% <ø> (+0.1%)	:arrow_up:
query	67.9% <ø> (-0.4%)	:arrow_down:
x	81.8% <ø> (-0.3%)	:arrow_down:

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