MongoDB 2.6 write commands have arrays called "documents", "updates", and "deletes" which contain the array of operations. These are absent from the command body with OP_MSG, and they're added as document sequences instead.
Introduce a new function mongoc_apm_command_started_get_document_sequences which returns these documents no matter what format of write command was sent to the server. The return value will be NULL if the command is not insert, update, or delete. Converse for command-succeeded events.
In libmongoc 1.9, when we send an OP_MSG with a document sequence to the server, simulate a command body that includes the document sequence as a BSON array in order to preserve the same API as OP_QUERY write commands.