rculfhash: extract compare_fct and hash_fct from the hash table

rculfhash: extract compare_fct and hash_fct from the hash table

Leave the compare_fct and hash_fct to the caller, which can therefore
become the only one which needs to know the layout of struct
cds_lfht_node (except for the _cds_lfht_node part).

This will enable us to remove the cds_lfht_node structure from the hash
table (moving it entirely to the caller), replacing it by
_cds_lfht_node, which is really the only part the hash table needs to
know about.

Use-cases that can benefit from this extra flexibility:

>> Sometimes the caller don't have the key or the keys which are big
>> are saved separated. Example: file server
>> Or key comparation is too slow.
>> Or caller just need approximate lookup.
>> Or caller want to compare keys with different strategies for
>> performance.

Suggested-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>