Addressed several GitHub issues.

[mhhen]

• Issue #25: Revised API so SDK owns payload buffers.
• Issue #24: Documented thread-safety for APIs.
• Issue #23: Changed use of field to be similiar to 2110-20.
• Issue #22: Added user_cb_param_ptr for payloads.
• Issue #18: Created JSON schema files and separated connection from payload.

[jmailhot]

based on the team meeting on 5/23 there is consensus to apply this PR. @mhhen please proceed.