Add lttng hash table support to liblttng-consumer

Add lttng hash table support to liblttng-consumer

Remove linked list usage from liblttng-consumer and replace them by
lockless RCU hash tables.

Note that there is still a mutex lock protecting those hash tables and
no RCU lock mechanism used. For now, it's OK and a very small
performance hit.

Signed-off-by: David Goulet <dgoulet@efficios.com>