[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

cdc_stream/session_config.h

@@ -56,6 +56,10 @@ struct SessionConfig {
 
   // Time to wait until running a manifest update after detecting a file change.
   uint32_t file_change_wait_duration_ms = 0;
+
+  // Ports used for local port forwarding.
+  uint16_t forward_port_first = 0;
+  uint16_t forward_port_last = 0;
 };
 
 }  // namespace cdc_ft