add SCP support

SCP is an experimental feature, we have our own SCP implementation
since we can't rely on scp system command to proper handle permissions,
quota and user's home dir restrictions. The SCP protocol is quite simple
but there is no official docs about it, so we need more testing and
feedbacks before enabling it by default.
We may not handle some borderline cases or have sneaky bugs.

This commit contains some breaking changes to the REST API.
SFTPGo API should be stable now and I hope no more breaking changes
before the first stable release.

scripts/README.md

@@ -154,12 +154,12 @@ Output:
 ]
 ```
 
-### Get SFTP connections
+### Get active connections
 
 Command:
 
 ```
-python sftpgo_api_cli.py get-sftp-connections
+python sftpgo_api_cli.py get-connections
 ```
 
 Output:
@@ -173,9 +173,11 @@ Output:
     "remote_address": "127.0.0.1:41622",
     "connection_time": 1564696137971,
     "last_activity": 1564696159605,
+    "protocol": "SFTP",
     "active_transfers": [
       {
         "operation_type": "upload",
+        "path": "/test_upload.gz",
         "start_time": 1564696149783,
         "size": 1146880,
         "last_activity": 1564696159605
@@ -185,12 +187,12 @@ Output:
 ]
 ```
 
-### Close SFTP connection
+### Close connection
 
 Command:
 
 ```
-python sftpgo_api_cli.py close-sftp-connection 76a11b22260ee4249328df28bef34dc64c70f7c097db52159fc24049eeb0e32c
+python sftpgo_api_cli.py close-connection 76a11b22260ee4249328df28bef34dc64c70f7c097db52159fc24049eeb0e32c
 ```
 
 Output: