keystore: revamp

[guillaumemichel]

Fixes #1112

Part of #1095

KeyStore Revamp

• Change indexing strategy
  • KeyStore3 (winner) implementation from DON'T MERGE: provider: keystore benchmark #1139
  • Each multihash is stored as its own key
  • Stored under the hex /-separated key: base/path/f/2/a/...
• Size() function that will be used for Stats
• Removing Garbage Collection logic
  • It should be managed by the user (see ipfs/kubo@b540fba)
• Batching Put and Delete operations

[guillaumemichel]

Superseded by #1142