gdbserver: use 'gdb::function_view' in 'find_*' and 'for_each_*'

Remove the templated versions of 'find_thread', 'for_each_thread' and
'find_thread_in_random' and replace the template function argument with
'gdb::function_view'.  The usage of 'gdb::function_view' produces less
cryptic messages on errors and documents well the types of the
parameters taken by the callback and its return type.

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