sqlfs_instance_count() to get number of active sqlfs instances/threads

Each sqlfs_t_init/sqlfs_open call creates an instance of sqlfs, which is
tracked internally.  This functions allows wrappers to get the number of
active instances to understand the number of active threads.  This is used
to check that an IOCipher VirtualFileSystem can be unmounted.

refs #3402 https://dev.guardianproject.info/issues/3402
zero out buffers that temporarily hold password/key data

refs #3402 https://dev.guardianproject.info/issues/3402