session: set fifo's session index before vcl attach

session: set fifo's session index before vcl attach

Current implementation may have timing issue:

1. The TCP 3-way-handshake is completed but the app is busy and
   does not process the CONNECTED event.
2. VPP receive some data from peer and try to send a RX event to
   VCL, but the fifo's client session index is not initialized yet.
   So this event will be sent to incorrect session 0.

Type: fix

Change-Id: I54f222232968bfaea5f4745284a1cfacc89482a9
Signed-off-by: liuyacan <liuyacan@corp.netease.com>