Arcyd: log remote io with contexts, for timing

Start timing remote io, to make it clear where the bottlenecks are.
Add context managers to abdt_logging for reading and writing that will
automatically record how long each operation takes.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
$ ./testbed/arcyd/test_shell.sh
    $ ./poke.sh
    $ ./accept.sh
    .. check log output ..