[EVE-35] Caching aggregates to avoid repeated loads

Proposal:

• Place the loaded aggregate instance in a cache
• When getting a command for that instance, get it from the cache instead of loading it from the stream
• Check the stream revision
• If it’s higher, read the stream tail

It can be done in a composable version of the AggregateStore.

_EVE-35

[ugumba]

#218 , #219 and #220 together add implicit support for IMemoryCache (if injected). I’ve only been able to run the basic tests, but the changes are also very basic… 😃

Tests in my own application indicate that more than 10 times as many (basic) commands can be processed in 1 second than without caching. With longer intervals, the difference should be exponentially better.

I’ve ignored composability, but I’ll look into it next - use of IMemoryCache must at least be optional.

[alexeyzimarev]

As I am currently working on subscriptions (split of physical subscription and consumer pipeline), it is now up for grabs. It’s relatively straightforward, and I think the good first step would be to add it at the lowest level (read of stream in the event reader). Composition would work there nicely.