Support for updating an AutoView's sorting block without updating it's grouping block

[gcox]

Related #222
Resolves #286

Adds setSorting:versionTag: to YapDatabaseAutoViewTransaction, which allows you to update the sorting block without the overhead of re-grouping all records in the view.

High points:

• resortView method
• new _handleChangeWithRowid:... method that is only interested in the sorting block
• Two new tests added that test updating just the sorting block.

I don't like the first half of the resortView method where it gathers the row ids. It shouldn't be a problem for most data sets, but loading all rowIds into memory, each boxed in an NSNumber doesn't feel good at all. This should be refactored to use a data structure with a predictable memory footprint, but I wanted to get something working first to make sure I wasn't missing any larger concepts.

The two _handleChangeWithRowid:... methods can also probably be refactored a bit to share some code. Again, want to make sure I haven't missed something important first.

[robbiehanson]

Awesome ! 👍

loading all rowIds into memory, each boxed in an NSNumber doesn't feel good at all

This may not be as much of a problem on 64-bit. If memory serves correctly, the runtime has a special optimization for boxing numbers, which allows it to skip much of the normal object allocation/deallocation routines. There's a good post about it here.

Another option would be to use C++ vectors. In fact, this is the approach I took back in the day when 32-bit was still super common. You can see it in YapDatabaseViewPage.

But everything is functional, so at this point it's just minor performance improvements.