Use leveldb2 database by default for new databases

[ajsutton]

PR Description

Switch default format for new databases to leveldb2. Does not affect existing databases which will continue to use RocksDB.

Also fixed deprecation warnings in ForkChoiceStrategyTest because I thought it was doing something specific and started there before investigating further - turns out it just needed the updates to InMemoryStorageSystemBuilder.

Fixed Issue(s)

fixes #3881

Documentation

• I thought about documentation and added the documentation label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[ajsutton]

Documentation wise on this one it may be useful to have some docs a) clarifying that existing databases are unaffected and continue to be supported, and b) explaining how to delete the db and resync if people do want to switch to leveldb now.

Better migration tools will be provided in future releases but for nodes using "prune" mode, the migration is quick and simple:

1. Download the latest finalized state curl -o state.ssz -H 'Accept: application/octet-stream' http://other-node:5051/eth/v1/debug/beacon/states/finalized
2. Stop teku
3. Delete the beacon directory under your data-path. Make sure not to delete the validator directory if one is present which contains your slashing protection data
4. Start teku with the --initial-state state.ssz option to start from the finalized state you just downloaded. A new leveldb database will be created and teku will start from that recent state and should be back in sync and validating within a couple of minutes.

Nodes running in "archive" mode will have to resync from genesis to convert at this stage. The migration tools we'll add in future releases will make it possibly to convert the database over without having to resync.

[rolfyone]

LGTM