Service get relevant users through getDB.

getDB returns a JSON stringified containing username as key and array of
products name as value. Users and products are obtained by joining the
tables as follows: first obtain all products the user offers/wants and
then obtain all users wanting/offering those products.

In order to test getDB from client, set params of invoke to an existing user in
db.

Signed-off-by: Andreea Hodea <hodea_andreea@yahoo.com>