Make Rdb_transaction_impl::get_trx safe to call from other threads

[laurynas-biveinis]

• Introduce Rdb_transaction_impl::m_rdb_tx_mutex to protect writes to
  m_rocksdb_tx[USER_TABLE] from the query thread and reads from other threads.
• To protect query thread writes, add critical sections to begin_rdb_tx and
  release_tx methods.
• Make get_rdb_trx lock this mutex before returning the RocksDB transaction
  pointer and introduce unlock_rdb_trx method to unlock this mutex.
• Call the above methods in Rdb_trx_info_aggregator::process_tran. Move
  unrelated code out of this critical section.