testbed/arcyd/profile_...: doc simple profiling

Add a testbed script to document how to do simple profiling for arcyd.

Test Plan:
$ ./precommit.sh

Make sure that the script works and is informative
$ ./testbed/arcyd/test_shell.sh
    $ cd ../arcyd
    $ arcyd stop
    .. invoke profile_arcyd.sh and follow instructions ..
Arcyd: BREAK, remove unused plugin manager for now

In preparation for multi-threading, reduce the amount of things that
need to be thread-safe by removing the unused plugin manager for now. We
can re-instate it if we need it later.

Test Plan:
$ ./precommit.sh
Arcyd: BREAK, remove arcyd dev-status-html

Prior to moving to a multi-process model, start removing functionality
that won't fit cleanly into the new model.

Remove the 'dev-status-html' command with the intent to replace it with
something that fits later.

Test Plan:
$ ./precommit.sh
$ git grep -l dev-status-html
Arcyd: BREAK, remove arcyd instaweb

Prior to moving to a multi-process model, start removing functionality
that won't fit cleanly into the new model.

Remove the 'instaweb' command with the intent to replace it with
something that fits later.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
    $ exit
$ git grep -l instaweb
Arcyd: BREAK, remove arcyd *-status-html

Prior to moving to a multi-process model, start removing functionality
that won't fit cleanly into the new model.

Remove the 'arcyd-status-html' and 'repo-status-html' commands with the
intent to replace them with something that fits later.

Test Plan:
$ ./precommit.sh
$ git grep -l statushtml
$ git grep -l status-html
$ ./testbed/arcyd/acceptance_tests.sh