Major changes of command processing for sessiond

Major changes of command processing for sessiond

This follows up a TODO added at commit 5e16da05 by me and Mathieu
Desnoyers about the command processing using a command context.

This patch introduce a command context data structure which contains all
the necessary information to process a command received by the session
daemon from the lttng client.

The entire session daemon is refactored with this addition. All
userspace trace action also. The global current_session pointer is
removed and is now in the command context struct. Some of the functions
for the buffer allocation are removed/changed.

This is the first step of getting the session daemon multi-threaded.

This patch also fix the -h bug of the lttng client introduce in the
commit 5e16da05. Minor variable renaming also occured.

Signed-off-by: David Goulet <david.goulet@polymtl.ca>