osd: Add Transaction::TransactionData for fast encode/decode

osd: Add Transaction::TransactionData for fast encode/decode

TransactionData wrap the following fields:
      __le64 ops;
      __le32 largest_data_len;
      __le32 largest_data_off;
      __le32 largest_data_off_in_tbl;
      __le32 pad; //make TransactionData multiple of uint64_t

This struct can encode/decode just by a single memcpy instead of many
encode/decode operations.

Change-Id: I56df78def43bd2b80b77be0825756e133434a6e6
Signed-off-by: Dong Yuan <yuandong1222@gmail.com>