Return cost components as json; add getter cdb2_get_info()

[adizaimi]

Include statement cost and extended cost as part of lastresponse so we avoid extra call to the db to get cost.

• Always return numeric value of cost
• If get_cost is set, return extended cost info--cost components
• cdb2sql --cost prints extended cost as json string using cdb2api getter function cdb2_get_info()

Example json extended cost from cdb2sql:

> cdb2sql adidb -cost 'select moredata from t2 where uid in (select uid from t1 where i=2)' | tail +2
{"Cost":32.40,"NumRows":1,
 "Components": [
   {"Type":"Table Scan","Table":"t1","NumFind":1,"NumNextOrPrev":1},
   {"Type":"Table Scan","Table":"t2","NumFind":1},
   {"Type":"Index Lookup","Index":0,"Table":"t2","NumFind":1,"NumNextOrPrev":1},
   {"Type":"Temp Index Lookup","NumFind":1,"NumPrevOrNext":1,"NumWrites":1 }
]}

Signed-off-by: Adi Zaimi [email protected]