session: fix race condition in fifo allocation

Under some timing conditions,VCL may receive CONNECTED/ACCEPTED
event before ADD_SEGMENT event.

Timing example:

2 threads call segment_manager_alloc_session_fifos() parallelly

         Thread 1                Thread 2

       sm read lock                 |
            |                       |
     try to alloc fifo =>failed     |
            |                       |
       sm read unlock               |
            |                       |
       sm write lock                |
            |                       |
       add segment                  |
            |                       |
       sm write unlock              |
            |                  sm read lock
            |                       |
            |               try to alloc fifo=>successful
       sm read lock                 |
            |                  sm read unlock
            |                       |
            |                 emit CONNECTED/ACCEPTED
   emit ADD_SEGMENT event
            |
       sm read unlock

This commit move ADD_SEGMENT notification under the protection
of the write lock in some scenarios.

Type: fix

Signed-off-by: liuyacan <liuyacan@corp.netease.com>
Change-Id: I25d5475c5e6d37cfccefa9506f6030c26ce8ee9b