Cache NJsonSchema properties in JsonReferenceResolver

[lahma]

There's high reflection cost the code path usually always hits JsonSchema type.

Using my custom large (1,3MB) swagger file.

Before

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
GenerateFile	151.2 ms	2.92 ms	7.22 ms	6000.0000	3000.0000	78.63 MB

After

Method	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated
GenerateFile	112.0 ms	2.12 ms	4.56 ms	7000.0000	3000.0000	500.0000	78.27 MB