Feature: Allow to configure instance event store max size

Hello.

It would be great if the value used for https://github.com/codecentric/spring-boot-admin/blob/master/spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/eventstore/ConcurrentMapEventStore.java#L52 (and related subclasses) could be configurable with a property rather than being hardcoded to 100.

In this way, if wanted, some memory can be spared by running the compaction more often.