Implemented a rudimentary (slow) serialize_onto_blob implementation.
Added inefficiently implemented deserialize_from_blob function.
Merge branch 'next' into sam_1431
Made internal_disk_backed_queue_t use write_onto_blob but not any sort of blob deserialization.
Merge branch 'next' into sam_1431
Made write_stream_t::write use MUST_USE.
Added buffer_group_write_stream_t type.
Removed unnecessary vector and string copying in serialize_onto_blob and deserialize_from_blob (but there still is unnecessary write_message_t copying).
Moved write_onto_blob implementation to serialize_onto_blob.cc.
Fixed comment refering to a vector_stream_t that happens to now be a string_stream_t.
Made kv_location_set use serialize_onto_blob.
Removed a ridiculous comment.
Made get_secondary_indexes_internal use deserialize_from_blob.
Made disk_backed_queue avoid copying when reading.
Merge branch 'next' into sam_1431

Conflicts:
	src/btree/secondary_operations.cc
	src/btree/secondary_operations.hpp
	src/containers/disk_backed_queue.hpp
	src/rdb_protocol/btree.cc
	src/rdb_protocol/protocol.cc
Fixed merge error and added drivers/python/ql2.proto to the .gitignore.
Removed out-dated RSI comment.
Merge branch 'next' into sam_1431
Made deserialize_from_group use guarantee_deserialization.
Changed deserialize_from_group deserialization error message.