conn: reuse memory in Receive()

[florianl]

Use sync.Pool to reuse memory to receive messages.

// on current master
$ go test -memprofile mem.orig.out .
// on this PR
$ go test  -memprofile mem.new.out .
$  go tool pprof -base=mem.orig.out mem.new.out
File: netlink.test
Type: alloc_space
Time: Jun 12, 2024 at 5:46pm (CEST)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for -1285.91MB, 53.01% of 2425.69MB total
Dropped 45 nodes (cum <= 12.13MB)
Showing top 10 nodes out of 24
      flat  flat%   sum%        cum   cum%
-1263.47MB 52.09% 52.09% -1263.40MB 52.08%  github.com/mdlayher/netlink.(*conn).Receive
   15.06MB  0.62% 51.47%    15.06MB  0.62%  github.com/mdlayher/netlink.newConn.func1
     -15MB  0.62% 52.08%      -15MB  0.62%  github.com/mdlayher/socket.rwT[go.shape.int]
  -14.50MB   0.6% 52.68%      -20MB  0.82%  github.com/mdlayher/socket.(*Conn).Recvmsg
  -10.50MB  0.43% 53.12%    -6.50MB  0.27%  golang.org/x/sys/unix.Recvmsg
    2.50MB   0.1% 53.01%   -12.50MB  0.52%  github.com/mdlayher/socket.(*Conn).Sendmsg
      -2MB 0.082% 53.09% -1265.40MB 52.17%  github.com/mdlayher/netlink.(*Conn).receive
       2MB 0.082% 53.01%    -6.50MB  0.27%  github.com/mdlayher/netlink.(*conn).Send
         0     0% 53.01%  -637.94MB 26.30%  github.com/mdlayher/netlink.(*Conn).Execute
         0     0% 53.01%  -634.47MB 26.16%  github.com/mdlayher/netlink.(*Conn).Receive

[florianl]

Closing in favor of #215 or #207. The implementation of both alternatives are less complex while achieving the reduction of the memory footprint of netlink applications.