converting hostnames to lowercase

Case-insensitive handling of hostnames in lib/hash.c was removed in this commit https://github.com/ganglia/monitor-core/commit/02524bf2f5485cfd4cddb66bd3280e5a08a2232c

This leads to "HOSTNAME" and "hostname" being two separate keys in the hashtable. 

Hostnames are converted to lowercase when the path to the rrd file is constructed in write_data_to_rrd (gmetad/process_xml.c) so the aggregated data will likely end up in the same .rrd file anyway. However, it's more efficient to handle the hostnames in a case-insensitive manner first.