scylla-gdb.py: scylla_task_histogram: expose internal parameters
scylla-gdb.py: scylla_task_histogram: expose internal parameters

Make all the parameters of the sampling tweakable via command line
arguments. I strived to keep full backward compatibility, but due to the
limitations of `argparse` there is one "breaking" change. The optional
positional size argument is now a non-positional argument as `argparse`
doesn't support optional positional arguments.
Added documentation for both the command itself as well as for all the
arguments.

scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla