Spring Data Couchbase 2.0 sets the default consistency to "update-after"
which is good for performance reason but can be quite confusing. Since
the team has decided to switch to "read-your-own-writes" in 2.1, Spring
Boot already offers the improved default right now.

This commit exposes an additional property that can be used to change
the Couchbase's default consistency.

Closes gh-5159