Remove default to NotBounded in FTS

Doing a search query results in a response like:

String value: rest_index: Query, indexName: ftsIdx1, err: bleve: QueryBleve validating consistency, err: unsupported consistencyLevel: not_bounded

(Same issue if we add our own SearchOptions with ScanConsistency set to request_plus)

Note that the json that gets sent is the following:
{"ctl":{"timeout":75000,"consistency":{"level":"not_bounded"}},"query":{"match":"SampleSubvalue68","prefix_length":0,"fuzziness":0}}
 

Whereas other SDKs do not send consistency unless its via consistentWith for at_plus.

For example, an equivalent search query from Java SDK3 sends 
{"match":"SampleValue83","query":{"match":"SampleValue83"}}
Also the only option provided by other SDKs is not_bounded, so the others should probably be removed?

https://github.com/couchbase/couchbase-net-client/blob/8f13db061dccc7a0297b2db1732161418067cb09/src/Couchbase/Search/SearchScanConsistency.cs