abdt_conduit: refactor, rename on_io_event()

Rename on_io_event() to on_remote_io_write_event(), as we'll want to add
a similar log for reads. This also makes it clearer that we're not
intending to log local io here, just the stuff that's potentially
visible to users.

Test Plan:
$ ./precommit.sh
abdt_conduit: refactor, rename set_io_log_path()

Rename set_io_log_path() to set_remote_io_write_log_path(), as we'll
want to add a similar log for reads. This also makes it clearer that
we're not intending to log local io here, just the stuff that's
potentially visible to users.

Test Plan:
$ ./precommit.sh
abdt_logging: refactor, rename private module vars

Update the _IO_LOG* variables in accordance with the
on_remote_io_write_event() rename.

Test Plan:
$ ./precommit.sh
abdt_logging: refactor, extract helper

Extract _on_remote_io_event() so that we may later re-use it for
on_remote_io_read_event().

Test Plan:
$ ./precommit.sh
Arcyd: prepare to log remote io reads to a file

Add a new global logging function alongside on_remote_io_write_event(),
except for read events. This will allow us to examine what remote read
events slow us down.

This has a completely different configuration path than
on_remote_io_write_event() as it is instead specified in abdt_fs. In a
later breaking change we'll change writes to be configured in the same
way.

Test Plan:
$ ./precommit.sh
Arcyd: log remote git reads

Make it easier to verify that cache behaviour is correct by logging
potentially expensive remote git reads.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
    $ ./poke.sh
    $ ./accept.sh
    $ cat ../arcyd/var/log/remote_io_reads