Watches: Reset the index if it goes backwards

According to the Consul docs https://www.consul.io/api/features/blocking.html

> Reset the index if it goes backwards. While indexes in general are monotonically increasing(i.e. they should only ever increase as time passes), there are several real-world scenarios in which they can go backwards for a given query. Implementations must check to see if a returned index is lower than the previous value, and if it is, should reset index to 0 - effectively restarting their blocking loop. Failure to do so may cause the client to miss future updates for an unbounded time, or to use an invalid index value that causes no blocking and increases load on the servers. Cases where this can occur include:
> * If a raft snapshot is restored on the servers with older version of the data.
> * KV list operations where an item with the highest index is removed.
> * A Consul upgrade changes the way watches work to optimize them with more granular indexes.