Generate proper API description for Map types using "additionalProperties"

[clementdenis]

This PR changes the way Cloud Endpoints handles Map types when generating API specs:

• Previously, maps were always described as JsonMap, making the API Explorer buggy, and the Java clients barely usable.
• The new behavior is to use "additionalProperties" in the JsonSchema, that work really well for String-keyed maps, and generate correctly typed Java clients.
• A flag system based both on env variables and system properties (see MapSchemaFlag) can be used to fine tune the new behavior.

This introduces backward incompatible changes to the generated Java clients, and the stricter validation of acceptable Map types can make some APIs fail to generate the spec. Legacy behavior can be reintroduced for both of these using flags in MapSchemaFlag (see FORCE_JSON_MAP_SCHEMA and IGNORE_UNSUPPORTED_KEY_TYPES).

A note about Map types with array-like values:

• This use case is properly supported by both the Discovery and Swagger formats, and works fine in the API explorer.
• However, it is not supported by the cloud-based client library generator (which is presumably base on this project), it will generate invalid Java code.
• So the default behavior is to keep using JsonMap schema for the array-valued maps, but it can be forced to use additionalProperties with the MapSchemaFlag.SUPPORT_ARRAYS_VALUES flag.

I previously opened an issue about this: https://github.com/cloudendpoints/endpoints-java/issues/142

[codecov-io]

Codecov Report

Merging #155 into master will increase coverage by 0.13%. The diff coverage is 91.8%.

@@             Coverage Diff              @@
##             master     #155      +/-   ##
============================================
+ Coverage     80.12%   80.26%   +0.13%     
- Complexity     1689     1711      +22     
============================================
  Files           157      158       +1     
  Lines          5629     5684      +55     
  Branches        735      748      +13     
============================================
+ Hits           4510     4562      +52     
- Misses          838      840       +2     
- Partials        281      282       +1

Impacted Files	Coverage Δ	Complexity Δ
...com/google/api/server/spi/config/model/Schema.java	100% <ø> (ø)	2 <0> (ø)	:arrow_down:
...e/api/server/spi/discovery/DiscoveryGenerator.java	88.93% <100%> (+0.09%)	64 <0> (+1)	:arrow_up:
...oogle/api/server/spi/swagger/SwaggerGenerator.java	77.91% <100%> (+0.67%)	59 <0> (+3)	:arrow_up:
.../com/google/api/server/spi/config/model/Types.java	90% <77.77%> (-2.69%)	32 <3> (+3)
...gle/api/server/spi/config/model/MapSchemaFlag.java	90.9% <90.9%> (ø)	4 <4> (?)
.../api/server/spi/config/model/SchemaRepository.java	92.42% <94.28%> (+0.26%)	38 <8> (+10)	:arrow_up:
.../server/spi/discovery/CommonPathPrefixBuilder.java	100% <0%> (+5.55%)	9% <0%> (+1%)	:arrow_up:

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[tangiel]

Looks like there is a conflict now (may have merged out of order)

[clementdenis]

I rebased from latest master (conflict was only in imports).

[clementdenis]

I fixed formatting and copyright issues.