Don't sync table views while iterating over them

We don't actually want to be doing this at all (adding or deleting items while
fast enumerating isn't supported), and it made looping over a query and
mutating items take quadratic time.

Speeds up fast enumerating and mutating 20k items on my 4S from 22 seconds to 5
seconds.

Updates #796.
Make the array tableview validation functions better named
Merge pull request #797 from realm/tg-enumerate-and-mutate

Don't sync table views while iterating over them