Add option for double scheduling

Allows nodes to be selected multiple times during selection for a
particular stage
Upgrade to Airlift 0.94
Move type names to their own class
Include table in "inputs" field even if list of columns is empty

Add an entry to the inputs field in the query endpoint (/v1/query/{id})
for each table that participates in the plan, regardless of whether
tablescan nodes reference any columns (e.g., COUNT(*) queries)

Fixes issue #1673
Update 0.76 release notes