Add module api for looking into INFO fields

- Add RM_GetServerInfo and friends
- Add auto memory for new opaque struct
- Add tests for new APIs

other minor fixes:
- add const in various char pointers
- requested_section in modulesCollectInfo was actually not sds but char*
- extract new string2d out of getDoubleFromObject for code reuse

Add module API for

src/server.c

@@ -3909,7 +3909,7 @@ void bytesToHuman(char *s, unsigned long long n) {
 /* Create the string returned by the INFO command. This is decoupled
  * by the INFO command itself as we need to report the same information
  * on memory corruption problems. */
-sds genRedisInfoString(char *section) {
+sds genRedisInfoString(const char *section) {
     sds info = sdsempty();
     time_t uptime = server.unixtime-server.stat_starttime;
     int j;