rgw: make bucket index pending op expiration configureable

rgw: make bucket index pending op expiration configureable

Bucket index operations are transactional with data object
manipulation. The operation is prepared by adding a pending operation
record. And when the data object side is complete, the bucket index
operation is committed.

If it fails to be committed, later bucket listings will compare the
pending ops with the current data object state and see whether it
completed or not and then either commit or expire the op. The time
span for expiration is currently hard-coded as 120 seconds (unless
overridden in the bucket header, which can happen during "bucket
check").

This commit allows that expiration time to be configured.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>