Port ReadContext from ThreadLocal to ScopedValue

[prdoyle]

Sadly, ScopedValue is incompatible with try-with-resources syntax, so this would mean we must change the syntax of every use of a ReadContext.

[prdoyle]

I don't love passing a lambda to a thing when try-with-resources would work. The lambda makes checked exceptions awkward, and is a barrier to JIT optimization compared with plain old code.

Having said that, read contexts are supposed to be coarse-grained, and are often done automatically (eg. when hooks are called, or in the Spring Boot ReadContextFilter) so the only code affected would be user-written code that explicitly gets a read context. Ideally that's not very common.

So maybe we move toward lambdas. Only thing is: I do want to support checked exceptions should the user want to use them.