Add type property to levelup and abstract-leveldown implementations

[vweevers]

A continuation of Level/subleveldown#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 (#58) which ultimately removes the need to unwrap.

Tasks

Add type

• levelup
  • PR or commit: Level/levelup#678
  • Release: 4.3.0
• deferred-leveldown
  • PR or commit: Level/deferred-leveldown#75
  • Release: 5.2.1
• encoding-down
  • PR or commit: Level/encoding-down@8a23848
  • Release: 6.3.0
• subleveldown (already had it)
• multileveldown
  • PR or commit: Level/multileveldown#15
  • Release: not ours
• level-js
  • PR or commit: Level/level-js#179
  • Release: 5.0.0
• leveldown (not needed now)
• memdown (not needed now)
• rocksdb (not needed now)

Use reachdown utility

• subleveldown (reachdown itself was adapted from code in subleveldown)
  • PR or commit: Level/subleveldown#75
  • Release: 4.1.3
• multileveldown
  • PR or commit: Level/multileveldown#15
  • Release: not ours
• level-bench
  • PR or commit: Level/bench@9f475d9
  • Release: 0.1.0
• level-test (in its tests)
  • PR or commit: Level/level-test@8350402
  • Release: pending

[ralphtheninja]

Awesome name reachdown :)

[vweevers]

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