[cdc_stream] [cdc_rsync] Add --forward-port flag (#45)

Adds a flag to set the SSH forwarding port or port range used for
'cdc_stream start-service' and 'cdc_rsync'.

If a single number is passed, e.g. --forward-port 12345, then this
port is used without checking availability of local and remote ports.
If the port is taken, this results in an error when trying to connect.
Note that this restricts the number of connections that stream can
make to one.

If a range is passed, e.g. --forward-port 45000-46000, the tools
search for available ports locally and remotely in that range. This is
more robust, but a bit slower due to the extra overhead.

Optimizes port_manager_win as it was very slow for a large port range.
It's still not optimal, but the time needed to scan 30k ports is
<< 1 seconds now.

Fixes #12

common/port_manager.h

@@ -51,14 +51,11 @@ class PortManager {
   // Reserves a port in the range passed to the constructor. The port is
   // released automatically upon destruction if ReleasePort() is not called
   // explicitly.
-  // |check_remote| determines whether the remote port should be checked as
-  // well. If false, the check is skipped and a port might be returned that is
-  // still in use remotely.
   // |remote_timeout_sec| is the timeout for finding available ports on the
-  // remote instance. Not used if |check_remote| is false.
+  // remote instance.
   // Returns a DeadlineExceeded error if the timeout is exceeded.
   // Returns a ResourceExhausted error if no ports are available.
-  absl::StatusOr<int> ReservePort(bool check_remote, int remote_timeout_sec);
+  absl::StatusOr<int> ReservePort(int remote_timeout_sec);
 
   // Releases a reserved port.
   absl::Status ReleasePort(int port);