Make lttng_directory_handle reference countable

Currently, the lttng_directory_handle API assumes that the caller has
exclusive ownership of the lttng_directory_handle. Directory handles
are passed by reference and populated by an "init" method and
finalized using the "fini" method.

In order to allow multiple references to a path or directory file
descriptor in follow-up patches, the API is moved to a model where
directory handles are dynamically allocated and released using the
"put" method.

No change in behaviour is intended by this change beyond adapting the
API.

The objective of the change is to make it easier to implement copy
operations of trace chunks that are configured to use an fd-tracker
and reduce the number of open file descriptors when
lttng_directory_handles are copied.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ibc8e97ea9e949adafef44533c30b61e3a9fa1f7d