Provide guidance for NATS streaming store selection

[ColinSullivan1]

We should provide some guidance on when to use which store.

Memory

• Pure speed
• Do not require at least once delivery guarantees. This may be for use cases where NATS streaming if being used fo it's features (e.g. replay, rate limiting) rather than persistence requirements, and the system can rebuild state upon failure.

File

• Require at least once delivery.
• High throughput requirements and moderate number of channels

Database

• Have strong requirements around data backup
• High number of channels
• Moderate/Low throughput requirements.

CC @kozlovic for comment.

Note, saving off this content here until the site is refactored (otherwise I'd have just submitted a PR).

[gcolliso]

@kozlovic did you have any additional information wrt when and which store to use?

I can add the above comments from Colin to the Supported Stores document https://nats-io.github.io/docs/nats_streaming/clustering/supported-stores.html