test/osd: Extend ceph_test_rados_io_sequence to support copy operations

test/osd: Extend ceph_test_rados_io_sequence to support copy operations

- Adds support for the I/O exerciser to handle sequences involving pairs of
objects, including a swap operation to tell it which object to treat as
active. This is supported in interactive mode.
- Adds an operation to copy data from one object to another using
copy-from. This is supported in interactive mode.
- Adds support for using the truncate operation to make an object bigger.
- Adds a new CLI argument (--object_copy) to specify a secondary object to use.
- Adds a new sequence (SEQUENCE15) to test copying from one object to another.

Signed-off-by: Adam Lyon-Jones <adamlyon@uk.ibm.com>