Since both submitting a command no longer requires a cancellable, and it
is desirable to avoid sending a queued command that has already been
cancelled beforehand, add a new `Command.should_send` Cancellable
property to specify if a command should (still) be sent or not, and stop
passing a cancellable to ClientSession when submitting commands.
Allow call sites to pass in existing cancellable objects, and thus also
add it it as a ctor property to the Command class and all subclasses.
Lastly, throw a cancelled exception in `wait_until_complete` if send
was cancelled so that the caller knows what happened.
Remove redundant cancellable argument from
`Imap.Client.command_transaction_async` and rename it to
`submit_command` to make it more obvious about what it does.
85e9046c71