Change output format of Gerrit review count gsql to JSON_SINGLE

In JSON mode records are output as JSON objects using the column names as
the property names, one object per line. In JSON_SINGLE mode the whole
result set is output as a single JSON object.

Bug: 52329
Change-Id: Ifbb8176fc6212d93a7e11398a0b9118a5c8ba0a4