Make list matrix work, yeah! This would serialize uids for the next query, which would then again return a matrix, so on and so forth.

BREAKING change: convert to JSON. Given we're not tracking uids for each query uid, this will cause problems for the result. So, have to re-structure stuff here.

Modified posting package to include a way to query multiple posting lists for a given attribute. Introduce a new task.fbs which would be used for query and result encoding.