peak fragmentation ratio removed as it is a confusing field for users and trivial to compute at hand now that there is peak memory information in INFO output

save peak memory usage as statistic and show it in INFO. Also a new INFO field was added showing the fragmentation ratio using the peak memory info.

removed check for zmalloc return NULL in createClient(). The check was misplaced, and zmalloc never returns NULL.

added minimal cluster section in INFO output. This is only useful to check if the instance is or not configured as a cluster node, all the other informations are accessible using the CLUSTER command.