The main class was getting unwieldly and too general. Dividing the fields
helps adding support for other operation types.
This commit leaves most data duplicated, later commits clean up the affected code.
The two cases are now separated. In switchover, the promotion and
demotion targets can swap connections between each other without worry.
In failover, the two connection lists must be merged semi-intelligently.
The slave connections of the two servers are now saved to the operation
descriptor object at the start of the operation. This allows slave status
updating during the operation.
