Swtich to use mysql_file_pread/pwrite instead of mysql_file_read/write while reading and writing to IO_CACHE wherever feasible

The idea here is to switch to use mysql_file_pread/mysql_file_pwrite instead of mysql_file_read/mysql_file_write so that there would be no need to do a seek everytime one reads or writes to a file.

This would help us remove the calls to mysql_file_seek (most of the calls) because pread rdoes not change the file offset and one can just provide the offset to pread and then we would read the data from that offset.