Add and support the new hash table library

Add and support the new hash table library

liblttng-ht/ is the library introduced for hash table management.

This library uses the URCU ht-shrink branch (not yet upstream) making
the lib completely lockless. Import git head URCU hashtables at commit:
91a75cc579698814e47877cc8927fcae1f573739

Note that urcu hash table files are copied from the urcu git ree to this
git tree waiting for them to be upstream and stable.

Old hash table files are removed from libcommon.

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