[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/asset_stream_config.h

@@ -48,18 +48,21 @@ class AssetStreamConfig {
   // Loads a configuration from the JSON file at |path| and overrides any config
   // values that are set in this file. Sample json file:
   // {
+  //   "service-port":44432
+  //   "forward-port-first":"44433"
+  //   "forward-port-last":"44442"
   //   "verbosity":3,
   //   "debug":0,
   //   "singlethreaded":0,
   //   "stats":0,
   //   "quiet":0,
   //   "check":0,
-  //   "log_to_stdout":0,
-  //   "cache_capacity":"150G",
-  //   "cleanup_timeout":300,
-  //   "access_idle_timeout":5,
-  //   "manifest_updater_threads":4,
-  //   "file_change_wait_duration_ms":500
+  //   "log-to-stdout":0,
+  //   "cache-capacity":"150G",
+  //   "cleanup-timeout":300,
+  //   "access-idle-timeout":5,
+  //   "manifest-updater-threads":4,
+  //   "file-change-wait-duration-ms":500
   // }
   // Returns NotFoundError if the file does not exist.
   // Returns InvalidArgumentError if the file is not valid JSON.