Pause indexes during initial / heavy sync

[hackergrrl]

Without this, as new data comes in, the main node process has to share time between indexing those new messages and adding new messages to local hypercores. kappa-core has a pause/resume API now, so it's just a matter of either

1. detecting "initial sync" (whatever that means in a p2p environment), or
2. detecting "lots" of messages arriving and pause indexes at some threshold until the rate drops back down