unable to restart uwsgi if upgrade apm version to 4.1.0

**Describe the bug**: ...
I faced below error when I start/restart uwsgi. The error occurs after I upgrade the package `elastic-apm` from `2.1.1` to the newest `4.1.0`. Can anyone help? Thanks a lot!
```
SIGINT/SIGQUIT received...killing workers...
worker 1 buried after 1 seconds
worker 2 buried after 1 seconds
worker 3 buried after 1 seconds
worker 5 buried after 1 seconds
worker 6 buried after 1 seconds
worker 7 buried after 1 seconds
Fri Mar 29 15:28:04 2019 - worker 4 (pid: 32580) is taking too much time to die...NO MERCY !!!
Fri Mar 29 15:28:04 2019 - worker 8 (pid: 32655) is taking too much time to die...NO MERCY !!!
worker 4 buried after 1 seconds
worker 8 buried after 1 seconds
goodbye to uWSGI.
```

**Environment (please complete the following information)**
- OS: [e.g. Linux]
- Python version: 2.7.5
- Framework and version [e.g. Django 2.1]: Django 1.9
- APM Server version: 4.1.0
- Agent version:
Linux version 3.10.0-327.36.3.el7.x86_64 (builder@kbuilder.dev.centos.org) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-4) (GCC) ) elastic/apm-agent-python#1 SMP Mon Oct 24 16:09:20 UTC 2016

**Additional context**

Here is my `uwsgi.ini`
```
[uwsgi]
uid = cdportal
gid = cdportal
pidfile = /var/run/uwsgi/uwsgi.pid
# stats = /var/run/uwsgi/stats.sock
cap = setgid, setuid
chdir = /var/www/cdportal/current/backend
module = cdportal.wsgi:application
master = True
vacuum = True
max-requests = 1000
daemonize = /var/log/uwsgi/cdportal.log
socket = 127.0.0.1:49152
workers = 8
threads = 30
listen = 1024
enable-threads = True
```