core-reducer: use memory-mapped I/O when loading cores

The reducer was trying to load whole core file into memory, giving the
OOM killer reason to terminate it. Using mmap decreases reducer's memory
footprint and is also subjectively faster.
Increase CORE_SIZE_LIMIT to 1GB

We can be more generous here as there still is a check against available
free space and when there would remain less than 50MB after the core is
saved, it is discarded:

if [ $((${freespace} - ${coresize_approx})) -lt 50000 ]; then
  omit_core=true
  logger "rich-core: dumping core might fill up disk - not dumping"
  ...