topdown: Insert serialized data in inter-query cache

Previously when inserting an item in the inter-query cache, only
the size of the raw string representing the HTTP response message body
was used to calculate the amount of memory used by the item. This was
errornous as the actual item inserted in the cache would hold not only the
raw string but also the JSON representation of the message body.

The deserialized JSON body would end up consuming as much as 20x more memory compared
to the JSON encoded version of the same data. As a result, the cache would end
up consuming more memory than the user-defined cache size limit.

This change serializes the data that will be inserted in the inter-query cache
so as to get a more accurate representation of the size of the cache.

Fixes: #3042

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>