Session layer improvements

Among others:
- Moved app event queue to shared memory segment
- Use private memory segment for builtin apps
- Remove pid from svm fifo
- Protect session fifo (de)allocation
- Use fifo event for session disconnects
- Have session queue node poll in all wk threads

Change-Id: I89dbf7fdfebef12f5ef2b34ba3ef3c2c07f49ff2
Signed-off-by: Florin Coras <fcoras@cisco.com>