Do not quote table names on Oracle. They appear to be already quoted.

Without this commit, Oracle complains as follows:
  [Doctrine\DBAL\DBALException]
  An exception occurred while executing 'INSERT INTO "oc_appconfig" ("""appid""",
  """configkey""", """configvalue""") VALUES (?, ?, ?)' with params ["core",
  "installedat", "1396972927.537"]:

  ORA-01741: illegal zero-length identifier
Add error handling for --clear-schema on Oracle.
Move PostgreSQL sequence resynchronisation out into PgSqlTools class.