Add support for partitioned queries

[flimzy]

Relates to go-kivik/kivik#448

[codecov]

Codecov Report

Merging #228 into master will increase coverage by 0.10%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
+ Coverage   91.49%   91.60%   +0.10%     
==========================================
  Files          31       31              
  Lines        2080     2131      +51     
==========================================
+ Hits         1903     1952      +49     
- Misses        126      127       +1     
- Partials       51       52       +1     

Impacted Files	Coverage Δ
dbstats.go	93.54% <91.30%> (-1.33%)	⬇️
db.go	82.57% <100.00%> (+0.25%)	⬆️
find.go	100.00% <100.00%> (ø)

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