Support using a common redis connection

If sentinel_host or sentinel_cluster_id is not provided it will try to
connect with a redis_host parameter

server/extension/test/0.5.0/sql/90_isochrone_test.sql

@@ -0,0 +1,7 @@
+-- Check for isochrone signatures
+SELECT exists(SELECT *
+              FROM pg_proc p
+              INNER JOIN pg_namespace ns ON (p.pronamespace = ns.oid)
+              WHERE ns.nspname = 'cdb_dataservices_server'
+              AND proname = 'cdb_isochrone'
+              AND oidvectortypes(p.proargtypes)  = 'text, text, geometry, text, integer[], text[]');