MB-20852 [7/N]: CheckpointManager::queueDirty: Pass vb by reference

MB-20852 [7/N]: CheckpointManager::queueDirty: Pass vb by reference

The VBucket argument to queueDirty is passed via a (reference to) a
ref-counted pointer. Nothing in this function modifies the arguments'
ref count, or passes it another owner; moreover it is also never null.

Therefore change to pass as a reference.

See also:
http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rr-smartptrparam

Change-Id: Iccfeb42922da558b6e9ab430b96829002e85af4a
Reviewed-on: http://review.couchbase.org/69016
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Jim Walker <jim@couchbase.com>
Tested-by: buildbot <build@couchbase.com>