Optimize AsyncJsonReferenceVisitorBase.Visit

[lahma]

• remove LINQ, reduce duplicate checks
• don't recurse strings or primitive types

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

Before

Method	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated
GenerateFile	106.9 ms	2.08 ms	2.56 ms	6500.0000	3500.0000	500.0000	74.28 MB

After

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
GenerateFile	86.33 ms	1.454 ms	1.215 ms	5000.0000	2000.0000	67.83 MB