Add support for recvmmsg and UDP GRO.

[brbzull0]

WIP
I will leave this as draft as I still need to :

• Make another look to the code and improve some stuffs.
• Ammend some stuffs and run some more tests and update the description, I still need to tweak some stuffs but the idea would be this.
• Make sure I have all CI green in the meantime.
  WIP

Descrition

This PR adds support for UDP GRO when reading the socket using recvm(m)gsg. This PR also includes support for recvmmsg when it is available by the OS.

There is now a new configuration variable:

# records.yaml
ts:
  udp:
    enable_gro: 1 # Enabled by default.

---

Notes

All the tests were done using GET only and setting the h2load parameter to --max-udp-payload-size=2k.

Feature	Total # Req	# Clients	# recvmmsg	# recvmsg
recvmmsg+gro	100k	100	22834	0
recvmmsg	100k	100	23501	0
recvmsg+gro	100k	100		247970
recvmmsg	100k	100		249097

As I didn't see much of a difference between having GRO enabled or disabled I did run a test with debug
enabled and I was able to count how many packets came with the GRO set in the ancillary data.
Then I realized why I wasn't actually seeing much difference, not too many packets were spliced by the kernel.

Packets with (No GRO/GRO) = Number of times that ATS detected a packet with spliced with GRO.

Feature	Total # Req	# Clients	# recvmmsg	# recvmsg	# Packets with (No GRO/GRO)	GRO %
recvmmsg+gro	100k	100	13929	0	223080 / 180	0.8%
recvmsg+gro	100k	100	0	278718	231760 / 34484	14%

Using recvmmsg reduced the number of syscall significantly

This is what I used for this tests:

bpftrace

bpftrace -e 'tracepoint:syscalls:sys_enter_recvm* { @[comm, probe] = count();  }' 

h2load

/opt/bin/h2load -n 100000 -c 100 --npn-list=h3 URL --max-udp-payload-size=2k

I was using only GET requests with around 15 header with about 80 bytes each.
I have not tried POST yet which I believe will give us better insights.