Client example: rework mutexes

Client example: rework mutexes

 * add a lot of commentary, including to the newly added
   example of passing values to the callback.

 * use a conditional variable instead of a single mutex.
   This is because glib mutexes (and their underlying
   default POSIX mutexes) have undefined behaviour when
   trying to lock an already owned mutex from the owning
   thread. As a side effect, this gets rid of the trylock
   mechanism which (even though it worked) was confusing.

 * fix the mutex being left locked on subsession call error.

Change-Id: I1afa973bcae57ff8c7434d2365d51893ff7696d0
Signed-off-by: Michal Bloch <m.bloch@samsung.com>