Close unused hypercores

[Frando]

Currently all hypercores (both kappa-record-db feeds and hyperdrive feeds) are open all the time. We can reduce this to feeds that either have active peers or are active writers. Other feeds can be closed an re-opened on demand.

This first has to be supported by kappa-record-db and then likely exposed in sonar-dat. It should help a lot with memory issues (and reaching the max opened files limit).