trace_processor: improve trace processor's iterator API

This CL makes three changes:
1. Adds a GetColumnName(uint32_t) method to the iterator. This allows
users of the iterator API to print out column names if they want to.
2. Simplifies the result of Next to a boolean. This allows users to
place iterator.Next() calls in the conditional of their for loop.
This also means GetLastError() should be called when Next() returns
false to check whether we had a true EOF or an erroneous EOF.
3. Removes IsValid(). This can be folded into Next() especially if
we have Next() being called in For loops.

Bug: 118867986
Change-Id: Ifd4ff64b236794a2b59ca0d5839d5072f590bcbd