Have hang analyzer write per-process gdb files

Prior to the work done in PM-1569, the hang analyzer would write separate debugger_mongod_<pid>.log files. This made it easy to search the thread stacks, etc. for a single process at a time. Now all of the gdb output across all mongod processes end up in a single debugger_mongod.log file and so cmd+f can jump between processes.

We should restore the old behavior of writing separate files with the `set logging file` command in gdb.