API: sync past blocks incrementally

[sohkai]

We currently fetch all past blocks in one shot, leaving us no way to tell a user how long they should wait and how far they have synced.

Given how the events caching works for externals, this is also problematic because we wait for all contracts' past events to load before committing to the app's cache.

From #206:

Something we could also do is detect if we're syncing a large amount of blocks (e.g. >100k) and then successively call eth_getlogs with to and from block numbers so that we don't have to wait for the entire operation to finish at once before we process any results.

As of today, loading the governance.aragonproject.eth org's voting app requires >5min to sync ~1million blocks, with the eth_getlogs sending 20kb of data all at once at the end. We could instead split this operation into 100k-1M block chunks to start loading data faster for the user