Implement OP_MSG section type 1 (document sequences)

AC:

• Add support to OpMsgRequest to have multiple sections.
• Add DocumentSequence type that contains a field name and an array of Documents
• On serialization of the sections in the OpMessageRequest, remove all DocumentSequences from the command and serialize them in subsequent sections as kind byte 1 -> int32 (size of the section in bytes) -> cstring (name of the field) -> documents (serialized as bson back to back with no separator)

Tests:

• Add unit tests on OpMessageRequest#toBin that test that when document sequences are present in the command they are properly serialized as multiple sections