sessiond: manage syscall table allocation using std::vector

sessiond: manage syscall table allocation using std::vector

Transitioning to the use of std::vector to manage the allocation and
resizing of the syscall table allows us to do away with memset() of the
syscall structure. The memset becomes problematic when lttng_ht_node_*
becomes non-POD in a follow-up change.

Change-Id: I94096782360f85ffffb0f72d188224af15e54d07
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>