Auto-calculate reasonable values for WriteBatchPoolSize and return batches to the pool even if commitlog queue is full

[richardartoul]

• Auto-calculate WriteBatchPool size based on commitlog queue size
• Return WriteBatch to pool even if commitlog queue is full
• Fix db.IsOverloaded() calculations

[codecov]

Codecov Report

Merging #1236 into master will decrease coverage by <.1%.
The diff coverage is 45.4%.

@@           Coverage Diff            @@
##           master   #1236     +/-   ##
========================================
- Coverage    70.8%   70.7%   -0.1%     
========================================
  Files         748     748             
  Lines       63055   63078     +23     
========================================
+ Hits        44644   44647      +3     
- Misses      15533   15546     +13     
- Partials     2878    2885      +7

Flag	Coverage Δ
#aggregator	81.6% <ø> (-0.1%)	⬇️
#cluster	85.6% <ø> (ø)	⬆️
#collector	78.4% <ø> (ø)	⬆️
#dbnode	80.8% <45.4%> (-0.1%)	⬇️
#m3em	73.2% <ø> (ø)	⬆️
#m3ninx	75.2% <ø> (ø)	⬆️
#m3nsch	51.1% <ø> (ø)	⬆️
#metrics	18.2% <ø> (ø)	⬆️
#msg	74.9% <ø> (ø)	⬆️
#query	60.3% <ø> (ø)	⬆️
#x	76% <ø> (ø)	⬆️

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[robskillington]

LGTM