Clean-up: sessiond: prepend `the_` to global variable names

Clean-up: sessiond: prepend `the_` to global variable names

This avoids name clashes between global variables and local variables or
function parameters (notification_thread_handle, for example). This is a
step towards enabling -Wshadow.

This also helps readability, in my opinion, as it helps quickly spot
that some code is using a global variable.

Change-Id: Ib0e35ad7efcc54fa88e1900cab3388b98a06b8d9
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>