This is likely the culprit behind several data corruption issues, e.g. where data has been
written to the freezer, but the deletion from pebble does not go through due to process
crash.
One difference between pebble and leveldb is that the latter returns error when performing Get on a closed database, the former does a panic. This may be triggered during shutdown (see #27237)
This PR changes the pebble driver so we check that the db is not closed already, for several operations. It also adds tests to the db test-suite, so the previously implicit assumption of "not panic:ing at ops on closed database" is covered by tests.
* cmd/utils, node: switch to Pebble as the default db if none exists
* node: fall back to LevelDB on platforms not supporting Pebble
* core/rawdb, node: default to Pebble at the node level
* cmd/geth: fix some tests explicitly using leveldb
* ethdb/pebble: allow double closes, makes tests simpler