Remove TCP endpoint goroutines.

This change removes all endpoint goroutines and all TCP processing is now done
inline in the TCP processor loop. TCP timers directly invoke handlers as
required rather than assert a waker.

UnlockUser is also simplified to just queue the endpoint to the processor
instead of trying to process segments inline. This allows us to centralize logic
for TCP state handling in the processor. This potentially could involve an extra
wakeup but now that endpoint goroutines do not exist this is not such a big
concern as in case of busy servers the processor goroutines will already be
running anyway.

This change also allows us to clean up S/R as now restoring a TCP endpoint does
not require restarting a goroutine and moving it to the right logical point but
only requires that we restart any timers that may have been running when the
save was done and restore any port bindings as required.

Endpoint.Release is now removed in favor of Endpoint.Abort by using Abort in
places where we use Endpoint.Release.

Updates #231

PiperOrigin-RevId: 442673015

pkg/sentry/socket/netstack/stack.go

@@ -444,6 +444,11 @@ func (s *Stack) IPTables() (*stack.IPTables, error) {
 	return s.Stack.IPTables(), nil
 }
 
+// Pause implements inet.Stack.Pause.
+func (s *Stack) Pause() {
+	s.Stack.Pause()
+}
+
 // Resume implements inet.Stack.Resume.
 func (s *Stack) Resume() {
 	s.Stack.Resume()