Implement support for v2 API

[axw]

Related: elastic/apm-server#1237

• Core v2 protocol (Core v2 protocol support #180)
• Unit test buffer size (More v2 unit tests #182)
• Unit test request size (More v2 unit tests #182)
• Reinstate schema validation unit tests (More v2 unit tests #182)
• Unit test early server response / unread body (Even more v2 tests #183)
• Unit test error flushing (Even more v2 tests #183)
• Track entity types in ring buffer, to update discarded stats (Record event type in ring buffer header #184)
• Reinstate ELASTIC_APM_DEBUG messages (ELASTIC_APM_DEBUG controls logging #185)
• Move spans to top level ( Report spans independently of transactions #188)
• Update API and configuration docs (Update docs for v2 config and API #190)
• Parse v2 error responses
• Update HTTP transport's timeout to better cater for streaming

[axw]

• Won't bother parsing the error responses for now. We just log them as-is.
• I'm going to see how we go with leaving the timeout as it is: from connection to response. Given a default request time of 10 seconds, this leaves 20 seconds for connection and server response, which should be fine.

[axw]

Tested opbeans-go, with #197. All looks good, closing this.