Prevent accessing freed XA-committed RocksDB transactions

[laurynas-biveinis]

With MyRocks DDSE, a XA transaction followed by a DDL statement in the same
connection results in the following:

• The XA transaction in RocksDB is committed by rocksdb_commit_by_xid, which
  then deletes the RocksDB transaction object.
• The DDL statement in the same thread accesses the deleted object.
• If it does not crash first, the next RocksDB commit on that transaction object
  fails due to the transaction being in unexpected state (COMMITTED instead of
  STARTED).

Fix by:

• No longer deleting transactions in rocksdb_commit_by_xid and
  rocksdb_rollback_by_xid, which was suitable only for crash recovery.
• Prevent memory leaks in crash recovery by making the recovered transaction
  pointer vector in rocksdb_recover public, and deleting all the transactions
  there in rocksdb_post_recover.
• New method Rdb_transaction::restart_if_committed_by_xid with implementation in
  Rdb_transaction_impl that syncs the MyRocks transaction state with RocksDB by
  restarting the RocksDB transaction if needed.

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