MB-20852 [2/N]: Convert queue_operation to scoped enum

MB-20852 [2/N]: Convert queue_operation to scoped enum

In preparation for adding new queue_op for setVBucketState, convert to
a typesafe C++11 scoped enum. Improve the documentation around
queue_op, and related classes Checkpoint, CheckpointCursor.

Also improve the output streaming (operator<<) for Checkpoint class.

Change-Id: I8f29b8e9e8915a68e31550b78bf3131b3737e2d7
Reviewed-on: http://review.couchbase.org/68160
Reviewed-by: Jim Walker <jim@couchbase.com>
Tested-by: buildbot <build@couchbase.com>