feat: switch default database backend from goleveldb to pebble

[DeVikingMark]

Description

• Changes default database backend from goleveldb to pebble for better performance
• Updates all configs and tests to use pebble by default
• Adds pebble support to database compaction tool
• Maintains backwards compatibility with existing goleveldb databases

Changes

• Changed default DBBackend to "pebble" in config
• Updated e2e tests and generators to use pebble
• Added pebble support to experimental-compact command
• Updated documentation

Closes #1596

Migration

Existing nodes will need to resync their chain to switch to pebble. No action required to continue using goleveldb.

Testing

• Run e2e tests
• Test database compaction with both goleveldb and pebble
• Verify node syncs with both database backends

[evan-forbes]

thanks for trying this out @DeVikingMark

it might be easier and it would be more valuable to do this after the upgrade to v0.38, since that's what we will use long term. It might even already be done there.

tests must be passing before we can merge 🙂