refactor and cleanup API config validation

[tangiel]

This change moves config validation out of JsonConfigWriter into ApiConfigValidator. JsonConfigWriter is mostly deprecated at this point, with no need for the older JSON API config with discovery and Swagger generation being created without it.

• Fixed a potential performance issue in which classes were re-validated constantly after each new Endpoint class registration. Now, SystemService.Builder triggers a validation of all APIs after class registration is finished.
• registerInternalService has been removed. BackendService is the only class that utilized this, and it has been removed.
• resolveAndUpdateServiceConfig has been removed. Dynamic datastore config reloads are no longer supported.
• Correctly allow byte arrays and datastore Blobs as parameter types. Both of these are serialized/deserialized as strings.
• Move checking of return type to ApiConfigValidator.
• Move checking of parameter name/resource field name conflicts to ApiConfigValidator.
• Fix a bunch of test cases that had invalid API methods. Most of these either had a string return type, which isn't allowed, or a byte array or Blob resource, which also isn't allowed.

[codecov-io]

Codecov Report

Merging #47 into master will increase coverage by -0.06%.

@@             Coverage Diff              @@
##             master      #47      +/-   ##
============================================
- Coverage     79.74%   79.68%   -0.06%     
============================================
  Files           153      153              
  Lines          5407     5411       +4     
  Branches        696      693       -3     
============================================
  Hits           4312     4312              
- Misses          821      829       +8     
+ Partials        274      270       -4

Impacted Files	Coverage Δ	Complexity Δ
...oogle/api/server/spi/tools/GenApiConfigAction.java	30.76% <ø> (-1.67%)	0 <ø> (-2)
...rver/spi/config/validation/ApiConfigValidator.java	77.58% <100%> (+2.42%)	0 <ø> (-55)	:x:
...server/spi/config/jsonwriter/JsonConfigWriter.java	67.66% <100%> (-0.95%)	0 <ø> (-56)
.../java/com/google/api/server/spi/SystemService.java	82.26% <100%> (-0.53%)	0 <ø> (-39)
...oogle/api/server/spi/swagger/SwaggerGenerator.java	76.55% <100%> (+0.22%)	0 <ø> (-51)	:x:
...ain/java/com/google/api/server/spi/TypeLoader.java	94.54% <75%> (-0.74%)	0 <ø> (-18)
...pi/server/spi/config/model/ApiParameterConfig.java	85% <ø> (-1.25%)	0% <ø> (-33%)
...e/api/server/spi/config/model/ApiMethodConfig.java	87.03% <ø> (-0.62%)	0% <ø> (-65%)

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