* Track active ssh sessions established via the web
Adds open session tracking in `web.Handler` and introduces a new
`web.Server` to prevent closing the web `http.Server` during
a graceful shutdown if there are still open sessions. From the
`http.Server/Shutdown` docs:
> Shutdown does not attempt to close nor wait for hijacked
> connections such as WebSockets. The caller of Shutdown should
> separately notify such long-lived connections of shutdown and wait
> for them to close, if desired.
Because the web sessions use WebSockets, `Shutdown` will not
wait for them to be idle before terminating. This used to work due
to the fact that web sessions dialed the Proxy SSH Port to establish the
SSH session. Which meant that all accounting of active user connections
by `sshutils/server.go` was accurate. But now that sessions are
established directly via the `proxy.Router` the web sessions were
unaccounted for.
The `web.Server` now mimics the behavior of `sshutils/server.go` for
Shutdown. Only once all the active web sessions have been terminated
will the underlying http.Server be shutdown.
Fixes#20227
