removing warnings related to serialVersionUID and unused objects

provide context abstraction to instantiate a context with info about
HttpRequest and tracing (trace id and span id).
provide handler that is able to store context per-thread to support
Web servers that handle each request in a dedicated thread.
provide support for ServletContainerInititializer by introducing
request context scope that can be "entered" and "leaved".

the empty instance of HttpRequest is used as a reference for empty object.

refactor Context constructor to set null instead of empty HttpRequest.
update traceparent context load method name and comments.
fix string comparison bug in traceparent context load method.

replace String.length() > 0 with !String.isEmpty().
strict W3C trace parent format validation

RequestContextScope is removed to avoid ambiguous use

move Context and ContextHolder to the root library package.
initialize ContextHolder to InheritableThreadLocal if configuration property is provided.
Add support for additional configuration property into LoggingConfig.

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