deproxy doesn't handle trailers with -T option

[RomanBelozerov]

Tempesta sends request and trailers separately if -T option is enabled. For request (dbg 6 for test t_frang.test_http_trailer_split_allowed.FrangHttpTrailerSplitLimitOnTestCase.test_accepted_request):

GET / HTTP/1.1
Host: debian
Transfer-Encoding: gzip, chunked
X-Forwarded-For: 127.0.0.1
via: 1.1 tempesta_fw (Tempesta FW pre-0.7.0)
Connection: keep-alive

4
test
0
HdrTest: testVal

1. Tempesta receives request without trailers and forwards it to server.
2. DeproxyServer receives request without trailers and parses it.

   
   Deproxy: SrvConnection: Receive request.
   GET / HTTP/1.1
   Host: debian
   Transfer-Encoding: gzip, chunked
   X-Forwarded-For: 127.0.0.1
   via: 1.1 tempesta_fw (Tempesta FW pre-0.7.0)
   Connection: keep-alive

4 test 0

3. DeproxyServer send response
```python
Deproxy: SrvConnection: Send response.
b'HTTP/1.1 200 OK\r\nContent-Length: 0\r\nConnection: keep-alive\r\n\r\n'

4. Tempesta receives trailers and forwards them to server.

       Deproxy: SrvConnection: Receive data.
   HdrTest: testVal

5. DeproxyServer recieves error

   Deproxy: SrvConnection: Can't parse message
   <<<<<
   HdrTest: testVal
   >>>>>

Please add:

• [ ] parsing trailers for DeproxyServer in handle_read;
• [ ] test for response with trailers and -T 1 option;