fix(filer): use DROP TABLE IF EXISTS in SQL stores (#10158)

Concurrent bucket deletion across multiple filer replicas races on the
per-bucket DROP TABLE. The first replica drops the table; the rest hit
an undefined-table error (postgres 42P01, mysql 1051) which propagates
out of DeleteFolderChildren and panics the filer. On restart the same
pending DROP re-runs and the filer crash-loops.

Make the drop idempotent. Same defect class in all SQL backends, so fix
postgres, postgres2, mysql, mysql2, and sqlite together.

weed/filer/postgres2/postgres2_store.go

@@ -70,7 +70,7 @@ func (store *PostgresStore2) initialize(createTable, upsertQuery string, enableU
 	}
 	gen := &postgres.SqlGenPostgres{
 		CreateTableSqlTemplate: createTable,
-		DropTableSqlTemplate:   `drop table "%s"`,
+		DropTableSqlTemplate:   `drop table if exists "%s"`,
 		UpsertQueryTemplate:    upsertQuery,
 	}
 	store.SqlGenerator = gen