Another overhaul of transaction/savepoint handling

Another overhaul of transaction/savepoint handling

- Move most of the transaction logic back to ::Storage, only leave
  DBI-specific overrides in ::Storage::DBI
- Fix bug where a nested rollback would destabilize the entire execution
  chain
- Better checks for connectivity/ordering of txn/svp operations
- Make DBIC::Storage::NESTED_ROLLBACK_EXCEPTION a proper exception subclass
- Standardize the txn/svp names - renamed the actual workers
  _dbh_begin_work               to _exec_txn_begin
  _dbh_commit                   to _exec_txn_commit
  _dbh_rollback                 to _exec_txn_rollback
  _svp_[begin|release|rollback] to _exec_svp_[begin|release|rollback]