Allow MAX_SEC_KEY_LEN to be a configurable value

The secondary key max length check value, MAX_SEC_KEY_LEN, appears to be a constant, which means that it is not possible to index documents that exceed this criteria.

It would be nice to make this value configurable so that users can raise it as necessary to ensure that no documents are skipped in the indexing process.

We already do this for the max_array_seckey_size value, which is not a constant and is configurable via the following command:

curl -X POST http://<hostname>:9102/settings -u <username:password> -d '{"indexer.settings.max_array_seckey_size": 10240}’