Expose more N1QL Query Options

During review of the possible n1ql query options with Marco we settled on exposing the following from the SDK directly in addition to the ones already present:

for fine-tuning the request performance (similar to max_parallelism which we expose already)

• scan_cap: Maximum buffered channel size between the indexer client and the query service for index scans. This parameter controls when to use scan backfill. Use 0 or a negative number to disable.
• pipeline_batch: Controls the number of items execution operators can batch for Fetch from the KV.
• pipeline_cap: Maximum number of items each execution operator can buffer between various operators.

also, if the user knows the request is only ever a select, for security reasons it can make sense to tell the server this thing is readonly and it will prevent mutations from happening.

• readonly: Controls whether a query can change a resulting recordset.
  If readonly is true, then the following statements are not allowed:

CREATE INDEX
DROP INDEX
INSERT
MERGE
UPDATE
UPSERT

Note that https://developer.couchbase.com/documentation/server/5.0/settings/query-settings.html has these params documented but the docs are wrong at the time of ticket creation (it shows - and not _ when describing the on the wire representation of the param)