feat: add composite sources support

[stepankuzmin]

This PR allows composing multiple table sources into one composite source. Table sources endpoints now support multiple tables in one request. E.g you can request tiles that contain data from multiple tables by listing the tables separated by a comma:

/{schema_name1}.{table_name1},{schema_name2}.{table_name2}.json
/{schema_name1}.{table_name1},{schema_name2}.{table_name2}/0/0/0.pbf

Using with Mapbox GL JS

map.addSource('points', {
  type: 'vector',
  url: `http://0.0.0.0:3000/public.red_points,public.blue_points.json`
});

map.addLayer({
  id: 'red_points',
  type: 'circle',
  source: 'points',
  'source-layer': 'public.red_points',
  paint: {
    'circle-color': 'red'
  }
});

map.addLayer({
  id: 'blue_points',
  type: 'circle',
  source: 'points',
  'source-layer': 'public.blue_points',
  paint: {
    'circle-color': 'blue'
  }
});

You can try this docker image with composite sources support and give any feedback on this

docker pull urbica/martin:pr-184

Relates: #36 #107

[codecov]

Codecov Report

Merging #184 (20005fc) into master (e953aa3) will increase coverage by 2.23%.
The diff coverage is 77.69%.

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
+ Coverage   41.14%   43.37%   +2.23%     
==========================================
  Files          13       14       +1     
  Lines        1541     1623      +82     
==========================================
+ Hits          634      704      +70     
- Misses        907      919      +12     

Impacted Files	Coverage Δ
src/lib.rs	34.88% <ø> (+0.34%)	⬆️
src/table_source.rs	65.21% <41.93%> (-15.03%)	⬇️
src/composite_source.rs	78.94% <78.94%> (ø)
src/server.rs	40.81% <89.28%> (+3.14%)	⬆️
src/dev.rs	100.00% <100.00%> (ø)
src/utils.rs	66.66% <100.00%> (+5.30%)	⬆️
... and 1 more

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