Fix agent cache

[wouterdb]

Problem

1. The agent cache is now version based, this is complicated for end-users and becomes difficult in the new agent design
2. Time based expiry is broken: https://github.com/inmanta/inmanta-core/issues/833

Proposed solution

1. we keep version open for a fixed time X (e.g. 1 minute after last use ended)
2. we generate a 'tick' to the cache regularly (e.g. 1s) to make it run finalizes), on a threadpool (taking care not to race actual usage.

[sanderr]

Discussed in planning meeting. Main difficulty that needs to be addressed: discarding an entry from the cache means calling a custom finalizer, which is not async, therefore has to be run on a thread. It must only be called when the agent is not actively using the thing cached. Might be easiest to extend this to "when the agent is not doing anything at all".

[sanderr]

See status on Slack