Issue with unstable throughput

Hi!
We downloaded boringtun and ran performance tests, and we found that throughput was inconsistent.
Here's a case.
for server:
```bash
# wireguard-go conf
root@n198-100-016:~# cat /etc/wireguard/wgx.conf
[Interface]
Address = 192.169.1.1/24
ListenPort = 5001
PrivateKey = oHtzn34+s2LB9o8bIFNrvUBTZ7VoXOOc5nXTZOs5jnk=

[Peer]
AllowedIPs = 192.169.1.3/32,192.168.1.3/32
PublicKey = FoGUDBjfh0wu73Ohii3hTVn0NrRxTN4hKeIOVhaaWVQ=
Endpoint = 10.198.100.17:5001
# wireguard-go cmd
wg-quick up wgx

# boringtun conf
root@n198-100-016:~# cat /etc/wireguard/wgu.conf
[Interface]
Address = 192.168.1.1/24
ListenPort = 5000
PrivateKey = oHtzn34+s2LB9o8bIFNrvUBTZ7VoXOOc5nXTZOs5jnk=

[Peer]
AllowedIPs = 192.168.1.3/32
PublicKey = FoGUDBjfh0wu73Ohii3hTVn0NrRxTN4hKeIOVhaaWVQ=
Endpoint = 10.198.100.17:5000

# boringtun cmd
su zhaoya
sudo WG_QUICK_USERSPACE_IMPLEMENTATION=boringtun wg-quick up wgu

iperf -s
```
for client:
```bash
# wireguard-go conf
root@n198-100-017:~# cat /etc/wireguard/wgx.conf
[Interface]
Address = 192.169.1.3/24
ListenPort = 5001
PrivateKey = sFWijh452FMN5krVhsF1wOoyK8A2aiEg8cUXJuWOX08=

[Peer]
AllowedIPs = 192.169.1.1/32,192.168.1.1/32
PublicKey = 4+b3CbE4qmzJbheQTm4pCC9Ksx8pnHbncGAmgC/oDjY=
Endpoint = 10.198.100.16:5001
PersistentKeepalive = 25

# wireguard-go cmd
wg-quick up wgx

# boringtun conf
root@n198-100-017:~# cat /etc/wireguard/wgu.conf
[Interface]
Address = 192.168.1.3/24
ListenPort = 5000
PrivateKey = sFWijh452FMN5krVhsF1wOoyK8A2aiEg8cUXJuWOX08=

[Peer]
AllowedIPs = 192.168.1.1/32
PublicKey = 4+b3CbE4qmzJbheQTm4pCC9Ksx8pnHbncGAmgC/oDjY=
Endpoint = 10.198.100.16:5000

# boringtun cmd
su zhaoya
sudo WG_QUICK_USERSPACE_IMPLEMENTATION=boringtun wg-quick up wgu

iperf -c 10.198.100.16 -i 1
```
for result:

We changed the architecture and created multiple reuseport sockets for each thread. Each thread operated only on its own socket and the performance was very stable.
Here's our patch:
[multi_poll.diff.txt](https://github.com/cloudflare/boringtun/files/8849104/multi_poll.diff.txt)

I don't know if this issue is known, and if the community has a fix or optimization plan.

Looking forward to reply.