Add `db.getMany(keys)` across the board

[vweevers]

Background: Level/abstract-leveldown#380 and Level/levelup#491.

• level-supports: Add features: getMany, keyIterator, valueIterator, iteratorNextv, iteratorAll supports#11 (2.0.1)
• abstract-leveldown: Add db.getMany(keys) abstract-leveldown#381 (7.2.0)
• leveldown: Add db.getMany(keys) leveldown#787 (6.1.0)
• memdown: Add db.getMany(keys) memdown#212 (6.1.0)
• level-js: Add db.getMany(keys) level-js#214 (6.1.0)
• deferred-leveldown: Add db.getMany(keys) deferred-leveldown#89 (7.0.0)
• encoding-down: Add db.getMany(keys) encoding-down#102 (7.1.0)
• levelup: Add db.getMany(keys) levelup#725 (5.1.0)
• subleveldown: Add db.getMany(keys) subleveldown#107 (6.0.0)
• multileveldown: Level/multileveldown@ff1ba48 (5.0.0)
• level-party: 5.1.0
• level-mem: 6.0.0
• level: Document new features level#207
• rocksdb: Add db.getMany(keys) rocksdb#184 (5.2.0)
• level-rocksdb: just docs I moved this remaining item to Sync README with level level-rocksdb#75