CDB-2891 Uses search_path if param exists for torque and mapnik queries
Using grainstore version with search_path support
cartodb,public in search_path has to be set in an upper layer. Also removing the workaround with dbname and search_path.
Removes search path changes as they are no longer needed
[CDB-3678] Set X-Served-By-Host response header
CDB-3678 Only add the header if api_hostname exists in environment
Merge pull request #194 from CartoDB/CDB-3678

[CDB-3678] Set X-Served-By-Host response header
Merge pull request #195 from CartoDB/CDB-2891-search_path

Use grainstore version with support for specifying pg search path