Explicitly initialize plugins (fixes #38)

[dlsniper]

This allows a user to import plugins on local development environments which do not have access to AWS metadata without stalling the process at boot time.

Each of the three plugins will need to be imported and initialized manually.
The API will safeguard against multiple requests but it's not goroutine safe.

[ScottMansfield]

why not surround the init code with sync.Once to make them concurrent safe?

[dlsniper]

why not surround the init code with sync.Once to make them concurrent safe?

That's a very good point, I'll update the PR to reflect this, thank you!

[docmerlin]

Yah, this is a better way of doing it. init forces it to be at starttime and can cause conflicts occasionally.

[dlsniper]

Yah, this is a better way of doing it. init forces it to be at starttime and can cause conflicts occasionally.

This is not why I wanted to change this, please read #38.

[luluzhao]

Could you please rename these three plugin package since directly changing the plugin behavior under the same package name will not result in any compile or runtime errors for consumers, and will silently fail to record AWS metadata on their segments. Like "github.com/aws/aws-xray-sdk-go/awsplugins/ec2"