Java SDK release V3.0.0

Hello,

This issue tracks next major version release changes : `V3.0.0`

## Remove X-Ray Dependency on AWS X-Ray V1.11 (#92)
X-Ray internally creates an AWS SDK X-Ray client to poll for sampling targets and sampling rules. We have substituted the AWS SDK client with a native HTTP client, and plan to remove all classes and APIs that reference the AWS SDK, along with the dependency itself, for v3.0.0.

## Apply sampling to segments created with beginSegment (#74)
**REQUIRES DISCUSSION:** Even though applying sampling by default is the correct behavior, many customers today are using `beginSegment` without expecting sampling to be applied. This change would not cause their code to fail to compile, since the API would remain the same, but could cause seriously unexpected, and very subtle, consequences. Since we introduced `beginSegmentWithSampling` as a choice for customers, we should consider not taking this action which could silently break existing code.

X-Ray currently has a separate API, `beginSegmentWithSampling`, to apply sampling to a new segment. But this will be done by the default `beginSampling` in v3.0, and we will deprecate the `beginSegmentWithSampling` API.

## Hide several internal/utility classes and constructors
The SDK today exposes several utility or internal implementation classes as `public`. Many of these are not intended for customer use and will be hidden in the next major version. An exact list of impacted classes will be made clear before the major version release.