gdbserver: add process specific thread list and map

Replace the servers global thread list with a process specific thread
list and a ptid -> thread map similar to 'inferior::ptid_thread_map' on
GDB side.  Optimize the 'find_thread' and 'find_thread_ptid' functions
to use std::unordered_map::find for faster lookup of threads without
iterating over all processes and threads, if applicable.  This becomes
important when debugging applications with a large thread count, e.g.,
in the context of GPU debugging.

Approved-By: Simon Marchi <simon.marchi@efficios.com>