[FTS] - increase scorch in-memory fieldId uint16 representation

In scorch, there's a representation of a fieldID using uint16.

For dynamic indexing situations where the field might be high-cardinality (like a UUID?), that might not be enough. Example:

  { "timestamp": "2018/10/31-01:02:03",

     "devicesSeen": {

         "789b40174c0f4213a919c5542ad1af45": "online",

         "839b4014cf3041293ac19a545d1a2f45": "online",

         "18bb400f1c4041329a19a542cd15af45": "online",

         "403411242c0f1239a19c4a542d51af45": "online",

         "12344cf04213a91c955d21a4af45": "online"

     }

}

This ticket is to capture the idea of changing that in-memory representation to something bigger.

The on-disk representation in scorch, btw, does not have the uint16 limitation.