Add basic retries

[alexeyzimarev]

There's a package to add retries using Polly to subscriptions (for retrying event handlers). However, we need more places with retries, and there's no need for complex policies that Polly supports.

The idea is to add some basic retry mechanism to the core project, so it can be used in, for example, AggregateStore that will retry reads and writes for transient store errors.