Shared locking (shared_timed_mutex, shared_mutex)

Specifies that ~future does not block except for async futures.

Renames shared_mutex to shared_timed_mutex to leave room for an untimed version. Merged with LWG2288 (SG1 Motion 2): shared_mutex renamed to shared_timed_mutex where LWG2288 added it..

Adds the untimed std::shared_mutex. Editor kept 'Shared timed mutex types' nested under 'Mutex requirements' rather than under 'Shared mutex types', for consistency with sibling subclauses..