Enhanced quick update utility

[benhutchins]

Pulling this out from #675. I think this would be a useful enhancement.

A suggestion would be to expand the .update() command to avoid having to do 3 functions instead of 1. For example, this is currently the update() command:

 await Users.primaryKey.update({
      hash: id,
      range: undefined,
      changes: {
        status: 'active'
      }
    });

An option would be to use the commands set, add, subtract, etc so that you could issue update() this way:

 await Users.primaryKey.update({
      hash: id,
      range: undefined,
      set: {
        status: 'active'
      },
      add: {
       logins: 1
      }
    });

In other words, these other new methods are added, and "changes" becomes "set" (but would still need to alias "changes" to "set" so it doesn't become a breaking change.) A person could use set, add, subtract together in a single call as above or just individually, examples:

 await Stats.primaryKey.update({
      hash: id,
      range: undefined,
      add: {
        daily_total: 1
      }
    });

 let payment= 50;
 await Accounts.primaryKey.update({
      hash: id,
      range: undefined,
      subtract: {
        remaining_balance: payment
      }
    });

This gives a simple pattern to convert into the actual sdk call. Since you've already used "increment" and "decrement" in other places, for conformity I guess it makes sense to use those instead of add/subtract, or at least alias them.

 let payment= 50;
 await Accounts.primaryKey.update({
      hash: id,
      range: undefined,
      decrement: {
        remaining_balance: payment
      }
    });

Originally posted by @ktwbc in #675 (comment)