Add way for SAI to disable row to token index so internal tables may leverage SAI

Internal tables tend to use LocalPartitioner and may not actually have murmur tokens but rather LocalPartitioner, which is variable length bytes tokens! For internal use cases we don’t always care about paging so don’t really need this index to function.

The use case motivating this work is for Accord, we wish to add a custom SAI index on the system_accord.commands#routes column. Since this logic is purely internal we don’t care about paging, but can not leverage SAI at this moment as it hard codes murmur tokens, and fails during memtable flush