Add lock_shared/unlock_shared to so_5::null_mutex_t

[eao197]

There is so_5::null_mutex_t thas has empty (no-op) methods lock and unlock. It can be used for a case when actual locking is not needed (for example, in single threaded environments).

It may be useful to have also empty (no-op) methods lock_shared and unlock_shared. It that case so_5::null_mutex_t can be used instead of std::shared_mutex when actual locking is not needed.