Use bbolt as snapshot store

[wraymo]

This PR is based on #7. Try to implement bbolt snapshot instead of file snapshot.

[casbin-bot]

@tangyang9464 @closetool @sagilio please review

[CLAassistant]

All committers have signed the CLA.

[wraymo]

Updated the changes. Please take a look

[nodece]

Good work!

[nodece]

Currently, we are using the file snapshot store, if we switch the bbolt as snapshot store, maybe this breaks user data, do you have idea to improve this?

[wraymo]

Currently, we are using the file snapshot store, if we switch the bbolt as snapshot store, maybe this breaks user data, do you have idea to improve this?

I'm not sure whether we can simply migrate file snapshots to bbolt, that is to read file snapshots and put key-value pairs into bbolt.

[nodece]

This is a good idea, I think we can add a flag here, default to file snapshot, if the user switches the bbolt snapshot, we need to check whether the file snapshot exists, if true, we need to migrate the file snapshot to bbolt snapshot.

[wraymo]

This is a good idea, I think we can add a flag here, default to file snapshot, if the user switches the bbolt snapshot, we need to check whether the file snapshot exists, if true, we need to migrate the file snapshot to bbolt snapshot.

I just added relevant code.