Provide a programmatic way to cancel

[ricellis]

One for consideration.
Currently if you are using the API and you call the backup or restore functions there is no way to abort/cancel. For example if I was to try and write a dashboard to control backup and restore using the API there is no functionality I could use to implement a cancel button. Since backup and restore tend to be long running processes potentially processing many GBs or more it seems like this could be a valuable addition.

Using the CLI the process can simply be sent a signal in order to terminate early.

• couchbackup/couchrestore version(s) that are affected by this issue.

1.4.0, 2.0.0-SNAPSHOT

• Node.js version.

Any

[ricellis]

Note that if/when this is attempted then (assuming #65 has also been completed) it should be possible to enable cancel/resume testing for API cases as well as CLI by adding a call to whatever cancellation mechanism is implemented from the test framework's backupAbort function.