Add support to expire objects when using BoltDB store

[abstractj]

What:

On Gatekeeper we support 2 kinds of store: Redis and Gatekeeper. Unfortunatelly BoldDB does not provide a built-in support to expire objects, which would be helpful when handling refresh tokens.

Why

• Even though there are several ways to mitigate by leaving this to sysadmins protecting the store into the filesystem. It's nice to add an extra layer of protection.

How

• One idea that we discussed [here|https://github.com/keycloak/keycloak-gatekeeper/pull/484#discussion_r393983241] is to implement a goroutine to periodically clean up BoltDB. But nothing is set in stone, plus, we need to discuss which kinds of store we really want to support.

Reference:

• KEYCLOAK-13367 - Add support to expire objects when using BoltDB store