Elastic-apm

* implemented de-dotting of tag names and context keys (353)
* wrote a quickfix for the boto3/botocore instrumentation (367)
* fixed an issue with psycopg2 and encoded strings (366)

* fixed another issue in the new v2 transport (351)

* fixed an issue with instrumenting redis-py 3.0+
* fixed a multithreading issue that occurs when using threaded workers (335)

**BREAKING** Version 4 of the agent implements a new wire protocol for communicating with
the APM Server. This format is only supported in *APM Server 6.5+*.

Further breaking changes:

* the undocumented `AsyncioHTTPTransport` has been removed.
* the `flush_interval` and `max_queue_size` settings have been removed.
* new settings introduced: `api_request_time` and `api_request_size`.
* Some settings now require a unit for duration or size. See documentation on
configuration for more information.
* The option to provide a custom date for exceptions and messages has been removed.

Other changes:
* on Python 3.7, use [contextvars](https://docs.python.org/3/library/contextvars.html) instead of threadlocals for storing
current transaction and span. This is a necessary precursor for full asyncio support. (291)

* fixed an issue with instrumenting redis-py 3.0+