bookkeeper client doesn't have any mechanism to cancel speculative requests

[sijie]

Problem

#209 introduced the speculative policy interfaces. However the interfaces don't problem any mechanism to cancel speculative requests. so PendingReadOp and ReadLastAddConfirmedAndEntryOp don't have a way to cancel speculative requests when they are done.

Related Issues: #1476

[sijie]

#1476 only avoid double releasing. but still we need to cancel the scheduled speculative tasks.