Add type property to levelup and abstract-leveldown implementations

[vweevers]

A continuation of https://github.com/Level/subleveldown/issues/34. cc @ralphtheninja

Background

Modules like subleveldown and multileveldown have to peel off layers (levelup, deferred-leveldown and/or encoding-down) from an input db to get to the inner db, which they then rewrap with those same layers.

That process ain't pretty, so I wrote reachdown to hide that ugliness. It currently has to feature-detect deferred-leveldown and encoding-down, and this may clash with another abstract-leveldown implementation at some point. To counter that, we can add a type property across the board:

MyLeveldown.prototype.type = 'my-leveldown'

Note: with that in place, the unwrapping (or even merely the fact that we have to unwrap) still isn't very clean, but IMO it's acceptable as a medium-term solution because there's a plan to merge level(up) functionality into abstract-leveldown (https://github.com/Level/community/issues/58) which ultimately removes the need to unwrap.

Tasks

Add type

• [x] levelup
  • [x] PR or commit: https://github.com/Level/levelup/pull/678
  • [x] Release: 4.3.0
• [x] deferred-leveldown
  • [x] PR or commit: https://github.com/Level/deferred-leveldown/pull/75
  • [x] Release: 5.2.1
• [x] encoding-down
  • [x] PR or commit: https://github.com/Level/encoding-down/commit/8a23848c0956879ad4f81d3a048b204131875d02
  • [x] Release: 6.3.0
• [x] subleveldown (already had it)
• [ ] multileveldown
  • [x] PR or commit: https://github.com/mafintosh/multileveldown/pull/15
  • [x] Release: not ours
• [x] level-js
  • [x] PR or commit: https://github.com/Level/level-js/pull/179
  • [x] Release: 5.0.0
• [x] leveldown (not needed now)
• [x] memdown (not needed now)
• [x] rocksdb (not needed now)

Use reachdown utility

• [x] subleveldown (reachdown itself was adapted from code in subleveldown)
  • [x] PR or commit: https://github.com/Level/subleveldown/pull/75
  • [x] Release: 4.1.3
• [ ] multileveldown
  • [x] PR or commit: https://github.com/mafintosh/multileveldown/pull/15
  • [x] Release: not ours
• [x] level-bench
  • [x] PR or commit: https://github.com/Level/bench/commit/9f475d91eb4c50bbda5255aa60901ea44111bb66
  • [x] Release: 0.1.0
• [ ] level-test (in its tests)
  • [x] PR or commit: https://github.com/Level/level-test/commit/835040255d00aa6a91066757d749fdf462ca0201
  • [ ] Release: pending

[ralphtheninja]

Awesome name reachdown :)

[vweevers]

Closing because the remaining items are external (multileveldown) or tests-only (level-test).