osd: new format for Transaction encode/decode

osd: new format for Transaction encode/decode

This patch add a new fixed size struct Transaction::Op to represent
all actions.

All coll and ghobject used by the transaction are keeped in two maps:
  coll:   map<coll_t, __le32> coll_index;
  object: map<ghobject_t, __le32> object_index;

And the Op struct use the map value(__le32) to refer coll and object,
so each coll and object is only need to encode once in the transaction.

Other variable-size fields(key/value/data) is encoded in bufferlist
data_bl.

Change-Id: I52b2fcd3217a6cb35de7b309a6dd74a99478feb2
Signed-off-by: Dong Yuan <yuandong1222@gmail.com>