Elastic-apm

**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+

* fixed an issue when logging messages that are not strings (295, 312)
* backported some documentation fixes

* fixed an issue with detecting names of wrapped functions that are partials (294)
* fixed a bug in Flask instrumentation that could appear together with FlaskAPI (286)

* added sanitization for `Set-Cookie` response headers (264)
* added instrumentation for the non-standard `Connection.execute()` method for SQLite3 (271)
* added "authorization" to list of sensitive keywords, to ensure that "Authorization"
HTTP headers are properly sanitized (275)
* taught the Logbook handler how to handle the `stack=False` option (278)
* fixed a race condition with managing the timer-send thread (279)