Add support for pebble database

[danlaine]

Context and scope

Right now avalanchego can only use leveldb or an in-memory database (memdb). This ticket is to add support for pebble.

Support should include:

• Adding a new pebble package underneath the database package
  • Adding a Database type in that package
  • Having that Database type implement database.Database
• Adding pebble as a valid value for config --db-type
• Unit tests (See database/leveldb/db_test.go for example)

See hypersdk for example of usage. go-ethereum also uses this package.

Discussion and alternatives

Open questions