Index Management REVIEW: prefix, condition

As per the feedback during REVIEW phase (https://github.com/couchbaselabs/sdk-rfcs/pull/35), the index management API has a few changes:

• watchPrimary removed from the watchIndexes parameters
• all methods are now prefixed with N1ql (as in createN1qlIndex)
• createN1qlIndex has a nullable Expression whereClause that allows to define a condition for the index. This is not available on the convenience varargs methods.
• the condition is represented on IndexInfo, empty string if not applicable