session: add session debug cli

session: add session debug cli

- add session debug cli to enable fine control of which event logs
are enable/disable with below syntax
  session debug {show | group <list> level <n>}
  list may be entered with a dash, "0-4"
  or it may be entered with a comma, "0,1,4"
- fix compilation errors when SESSION_EVT is enable
- change SESSION_EVT_FREE_HANDLER to use DEC_SESSION_ED instead of
DEC_SESSION_ETD because the transport may already be free when the
handler is called

Type: improvement

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: Iab2989e0a847bb59002ef16494eebcc1d112b2ae