This PR makes it possible to modify the flush interval time via RPC. On one extreme, `0s`, it would act as an archive node. If set to `1h`, means that after one hour of effective block processing time, the trie would be flushed. If one block takes 200ms, this means that a flush would occur every `5*3600=18000`  blocks -- however, if the memory size of the cached states grows too large, it will flush sooner. 

Essentially, this makes it possible to configure the node to be more or less "archive:ish", and without restarting the node while reconfiguring it.