Upgrade to level packages v2.x

[Jacopo Scazzosi]

@vweevers is working on version 2 of Level packages, bringing in significant changes including dropping callbacks in favor of promises. abstract-level v2.0.0 is already on NPM and the Upgrade guide now details the changes between v1.x and v2.x.

Before upgrading we need to wait for the following packages to be updated to abstract-level v2:

• classic-level
  • work towards v2 happening in https://github.com/Level/classic-level/tree/v2
• memory-level
  • no branch for v2 yet
• browser-level
  • no branch for v2 yet
• @nxtedition/rocksdb
  • no branch for v2 yet
  • should ask @ronag an whether they intend to align with abstract-level v2

We should also study the potential performance impacts of moving to promise-based iteration techniques, which I suspect might make for a slightly different tradeoff between performance (slower) and maintainability (greater).