Add support for GET /_dbs_info

[flimzy]

No description provided.

[codecov]

Codecov Report

Attention: 77 lines in your changes are missing coverage. Please review.

Comparison is base (6c7e12e) 77.37% compared to head (ac369b9) 76.98%.

Files	Patch %	Lines
kivik.go	14.81%	23 Missing ⚠️
mockdb/clientexpectations_gen.go	0.00%	22 Missing ⚠️
couchdb/dbstats.go	0.00%	18 Missing ⚠️
mockdb/client_gen.go	0.00%	10 Missing ⚠️
mockdb/clientmock_gen.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #854      +/-   ##
==========================================
- Coverage   77.37%   76.98%   -0.39%     
==========================================
  Files         205      205              
  Lines       14776    14874      +98     
==========================================
+ Hits        11433    11451      +18     
- Misses       2848     2928      +80     
  Partials      495      495              

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