Privatize lttng-sessiond-comm

With the intent of keeping libust, which is really an instrumentation
library, stand-alone, we prefer not to make it depend on a library from
the lttng-tools tree. Therefore, we privatize lttng-sessiond-comm, and
both package will interact through a communication protocol rather than
a shared library.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>