Client: change several waitfor_* to use Context list

Client: change several waitfor_* to use Context list

Change waitfor_caps, waitfor_safe and waitfor_commit to Context list.

To make a non-blocking version of fsync (to be used for non-blocking write
and commit), we need to be able to signal an arbitrary Context on completion
of either of these lists.

add_nonblocking_onfinish_to_context_list Adds such a Context to the list.

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>