Fix ctx_thread statistics, set output buffers

0. Fix bug setting num_kmers_with_paths when threading reads
1. Open all files using futil_[gz]open_output() to set buffers
2. use DEFAULT_IO_BUFSIZE as global input buffer size
3. Added print_stats functions:
 gpath_set_print_stats()
 gpath_store_print_stats()
 gpath_hash_print_stats()