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.
Adds a sindex_status query.

Next step is to use it in the query language.
Adds sindex_status method to the table_t val.
Adds QL support for sindex_status and sindex_wait.
Sindex statuses seem to be sort of working.
Adds a method to get all dispatchees in the broadcaster.
In theory this makes sindex status go to all shards.