Use auto-implemented properties

[Joy-less]

This eliminates three instances of variable shadowing and slightly simplifies the code.

[bitfaster]

Have you verified this produces identical IL and assembly code across .NET Framework and .NET Core?

I have seen in the past that changing to auto properties or changing field order can result in the JIT changing memory layout resulting in regressions that use a lot of time to track down.

[Joy-less]

On .NET 6.0:

Before:

After:

On .NET Standard 2.0:

Before:

After:

I used ildasm.exe on BitFaster.Caching.dll. Looks to be the same memory layout to me, just the backing fields have different names.

[coveralls]

coverage: 99.168% (+0.07%) from 99.102%
when pulling 44df0a1 on Joy-less:use-auto-implemented-properties
into 55ac07e on bitfaster:main.