Don't load all of about-self into memory at startup

[staltz]

This is an idea that concerns about-self, but in general applies to any leveldb index where each KV is a reduced state of somehow (e.g. about-self, ssb-friends, etc).

We need to apply processRecord on the previous state, so that's why we load all the states on startup time. But this costs almost 1sec during startup. 1 second times N "state-sensitive indexes" is N seconds.

Alternatives:

• Load all states at startup time
  • This is what we currently do
  • Good for cases when there will be tons of processRecords called in succession, e.g. initial indexing
  • Bad for cases where the index is large and there won't be many processRecords afterwards, e.g. opening up the SSB app every day
• Load each state just-in-time
  • Such as loading it inside processRecord, right before pushing to the batch array
  • Good for cases when there are few processRecord calls, e.g. every day SSB usage
  • Bad for initial indexing
• Load up everything as soon as we detect lots of processRecord calls
  • Otherwise use the JIT technique
  • Might be a compromise, but seems tricky to implement, and there could be corner cases we don't see yet

Overall the question is how to load and update indexes which are state sensitive.