Serverless POC: Send all telemetry into a single payload

[obenkenobi]

Description

Due to the current implementation of the DataSender class, telemetry data (such as events, metrics, etc) are all written to file and console in seperate payloads. According to the serverless spec, the data should be merged into a single continuous payload.

Acceptance Criteria

Update the implementation of DataSenderServerlessImpl to save data into a temporary buffer.
Then add a new method to save tha data into the console & file and flush out the buffer.

[Newrelic-Boomi]

https://new-relic.atlassian.net/browse/NR-499526