Add rudimentary server mode for use in kivik cli tool

[flimzy]

No description provided.

[codecov]

Codecov Report

Attention: 41 lines in your changes are missing coverage. Please review.

Comparison is base (9c2784b) 78.91% compared to head (53179f6) 78.97%.

Files	Patch %	Lines
server/middleware.go	34.61%	14 Missing and 3 partials ⚠️
server/server.go	92.22%	9 Missing and 6 partials ⚠️
x/fsdb/db.go	0.00%	4 Missing ⚠️
x/fsdb/fs.go	57.14%	1 Missing and 2 partials ⚠️
server/errors.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #837      +/-   ##
==========================================
+ Coverage   78.91%   78.97%   +0.06%     
==========================================
  Files         166      169       +3     
  Lines       13414    13646     +232     
==========================================
+ Hits        10585    10777     +192     
- Misses       2390     2419      +29     
- Partials      439      450      +11     

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