support multipart responses in query

[adameat]

• it begins with version 7 for "/viewer/query".
• it toggles with schema=multipart (and it also requires to have Accept: multipart/x-mixed-replace).
• the return format is almost identical to "schema=multi".
• it allows to specify "output_chunk_max_size". it in bytes, and has no default, I suggest to set it to 1000 for now... maybe to 10000. let's experiment with that.
• it also allows to specify "concurrent_results" - but I suggest to use it only for debugging for now.
• it generates query_id automatically, so no need to specify it.
• and it cancels query on connection closure, so it could be easier to cancel running queries (but classic method should work as well).

[antonkovalenko]

#1742