WIP: Implementing TTL for cached datasource instances

[iwysiu]

What this PR does / why we need it:
Investigation for implementing caching for datasource instances. It's pretty simple and straightforward. I did this using a popular caching library that we already use in core grafana.

There are a couple minor implementation notes/questions:

• The linked issue mentions expiring after a certain amount of inactivity, this automatically expires instances after a set amount of time. I decided to do this because tracking the activity would require us to take a write lock every time we get an instance so that we could update it and would require us to write our own janitor processes.
• Both the InstanceManager consumers in this repo generate string keys, but updating the interface to expect one is technically a breaking change, so I didn't make it. We could make a new InstanceManagerWithExpiration type if we wanted to avoid the sprintf
• Would we want this behavior behind a feature flag?
• The chosen expiration timeouts were arbitrary, we could make them settable, but I don't think we need that.

Which issue(s) this PR fixes:

Fixes https://github.com/grafana/data-sources/issues/329

Special notes for your reviewer: