speculative workaround for high unclosed connection count between com…

…pute smgr & pageserver (#523)

One possible explanation for our high ESTABLISHED connection count
between computes and pageserver's page_service.rs is that we kill the
VMs before the FIN/RST packets from the `pg_ctl -m stop` leave the VM's
kernel.

The sleep added in this PR would give them a good chance.

Roll this out and observe.

Context: https://neondb.slack.com/archives/C036U0GRMRB/p1694427077510779?thread_ts=1694425364.531029&cid=C036U0GRMRB

neonvm/tools/vm-builder/main.go

@@ -260,6 +260,13 @@ action=/neonvm/bin/powerdown
 	scriptPowerDown = `#!/neonvm/bin/sh
 
 su -p postgres --session-command '/usr/local/bin/pg_ctl stop -D /var/db/postgres/compute/pgdata -m fast --wait -t 10'
+# The poweroff below is the busybox poweroff which goes straight to the kernel, i.e., LINUX_REBOOT_CMD_POWER_OFF / RB_POWER_OFF.
+# Our experiments have shown that, generally, this type of hard shutdown will not FIN/RST existing TCP connections (i.e., state ESTABLISHED).
+# Now, for the particular case of NeonVM, we just did 'pg_ctl stop'.
+# But, the libpagestore.c is currently not explicitly shutting down the TCP connection to pageservers, relying on the kernel
+# to implicitly close the socket on exit.
+# This sleep here is to give the kernel time to send the FIN/RST.
+sleep 1
 /neonvm/bin/poweroff
 `